test: Fix function does not return a value warnings

Many test stub functions are meant to return a value, but weren't. Clang
would generate a warning for each instance where we weren't returning
anything in a function that was meant to return a value.

    warning: non-void function does not return a value [-Wreturn-type]

For a specific example, my_radio::time_on_air() is supposed to return a
uint32_t, but wasn't returning anything. We'll return a zero instead of
relying on undefined behavior.

Without this, clang 11.0.1 was generating a virtual function
implementation with a `ud2` instruction to abort at run-time, causing
some execution of some unit tests to abort.

    Running main() from gmock_main.cc
    [==========] Running 10 tests from 1 test suite.
    [----------] Global test environment set-up.
    [----------] 10 tests from Test_LoRaPHYUS915
    [ RUN      ] Test_LoRaPHYUS915.constructor
    [       OK ] Test_LoRaPHYUS915.constructor (0 ms)
    [ RUN      ] Test_LoRaPHYUS915.restore_default_channels
    [       OK ] Test_LoRaPHYUS915.restore_default_channels (0 ms)
    [ RUN      ] Test_LoRaPHYUS915.rx_config
    [       OK ] Test_LoRaPHYUS915.rx_config (0 ms)
    [ RUN      ] Test_LoRaPHYUS915.tx_config
    Process 35669 stopped
    * thread ARMmbed#1, name = 'lorawan-loraphy-', stop reason = signal SIGILL: privileged instruction
        frame #0: 0x0000000000276f73 lorawan-loraphy-us915-unittest`my_radio::time_on_air(this=0x0000000800c2b048, modem=MODEM_LORA, pkt_len='\0') at Test_LoRaPHYUS915.cpp:90:5
       87       };
       88
       89       virtual uint32_t time_on_air(radio_modems_t modem, uint8_t pkt_len)
    -> 90       {
       91       };
       92
       93       virtual bool perform_carrier_sense(radio_modems_t modem,
    (lldb) disassemble --pc
    lorawan-loraphy-us915-unittest`my_radio::time_on_air:
    ->  0x276f73 <+67>: ud2
        0x276f75:       int3
        0x276f76:       int3
        0x276f77:       int3
    (lldb)

Author: Patater

UNITTESTS/stubs/FileHandle_stub.h

@@ -67,6 +67,7 @@ class FileHandle_stub : public FileHandle {
 
     virtual int close()
     {
+        return 0;
     }
 
     virtual short poll(short events) const

UNITTESTS/stubs/connectivity/AT_CellularDevice_stub.cpp

@@ -56,6 +56,7 @@ ATHandler *AT_CellularDevice::get_at_handler()
 
 CellularContext *create_context(const char *apn)
 {
+    return NULL;
 }
 
 void delete_context(CellularContext *context)

UNITTESTS/stubs/connectivity/LoRaMac_stub.cpp

@@ -94,6 +94,7 @@ void LoRaMac::post_process_mlme_ind()
 
 lorawan_time_t LoRaMac::get_current_time(void)
 {
+    return 0;
 }
 
 rx_slot_t LoRaMac::get_current_slot(void)

UNITTESTS/stubs/drivers/BufferedSerial_stub.cpp

@@ -85,6 +85,7 @@ ssize_t BufferedSerial::write_unbuffered(const char *buf_ptr, size_t length)
 
 bool BufferedSerial::hup() const
 {
+    return false;
 }
 
 void BufferedSerial::wake()

UNITTESTS/stubs/events/equeue_stub.c

@@ -105,7 +105,7 @@ void equeue_background(equeue_t *queue,
 
 int equeue_chain(equeue_t *queue, equeue_t *target)
 {
-
+    return 0;
 }
 
 int equeue_call_in(equeue_t *q, int ms, void (*cb)(void *), void *data)

UNITTESTS/stubs/storage/EmulatedSD.cpp

@@ -62,6 +62,7 @@ int EmulatedSD::deinit()
 {
     fclose(_p->fs);
     _p->fs = nullptr;
+    return 0;
 }
 
 int EmulatedSD::read(void *buffer, bd_addr_t addr, bd_size_t size)

connectivity/cellular/tests/UNITTESTS/framework/AT/at_cellularcontext/at_cellularcontexttest.cpp

@@ -153,6 +153,8 @@ class my_AT_CTXIPV6 : public AT_CellularContext {
         if (!_stack) {
             _stack = new my_stack(_at, *get_device());
         }
+
+        return _stack;
     }
     virtual uint32_t get_timeout_for_operation(ContextOperation op) const
     {

connectivity/lorawan/tests/UNITTESTS/features/lorawan/loramac/Test_LoRaMac.cpp

@@ -537,7 +537,7 @@ TEST_F(Test_LoRaMac, post_process_mlme_ind)
 
 uint8_t batt_cb()
 {
-
+    return 100;
 }
 
 TEST_F(Test_LoRaMac, set_batterylevel_callback)

connectivity/lorawan/tests/UNITTESTS/features/lorawan/loraphy/Test_LoRaPHY.cpp

@@ -88,6 +88,7 @@ class my_radio : public LoRaRadio {
 
     virtual uint32_t random(void)
     {
+        return 4;
     };
 
     virtual uint8_t get_status(void)
@@ -105,6 +106,7 @@ class my_radio : public LoRaRadio {
 
     virtual uint32_t time_on_air(radio_modems_t modem, uint8_t pkt_len)
     {
+        return 0;
     };
 
     virtual bool perform_carrier_sense(radio_modems_t modem,

connectivity/lorawan/tests/UNITTESTS/features/lorawan/loraphyas923/Test_LoRaPHYAS923.cpp

@@ -71,6 +71,7 @@ class my_radio : public LoRaRadio {
 
     virtual uint32_t random(void)
     {
+        return 4;
     };
 
     virtual uint8_t get_status(void)
@@ -88,6 +89,7 @@ class my_radio : public LoRaRadio {
 
     virtual uint32_t time_on_air(radio_modems_t modem, uint8_t pkt_len)
     {
+        return 0;
     };
 
     virtual bool perform_carrier_sense(radio_modems_t modem,