[flang][driver] Refactor one unit-test case to use fixtures (nfc)

Move the unit test from InputOutputTest.cpp to FrontendActionTest.cpp
and re-implement it in terms of the FrontendActionTest fixture. This is
just a small code clean-up and a continuation of:
  * https://reviews.llvm.org/D93544

Moving forward, we should try be implementing all unit-test cases for
Flang's frontend actions in terms of FrontendActionTest.

Reviewed By: sameeranjoshi

Differential Revision: https://reviews.llvm.org/D94922

flang/unittests/Frontend/CMakeLists.txt

@@ -1,6 +1,5 @@
 add_flang_unittest(FlangFrontendTests
   CompilerInstanceTest.cpp
-  InputOutputTest.cpp
   FrontendActionTest.cpp
 )
 

flang/unittests/Frontend/CompilerInstanceTest.cpp

@@ -9,7 +9,7 @@
 #include "flang/Frontend/CompilerInstance.h"
 #include "flang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Basic/DiagnosticOptions.h"
-#include "llvm/Support//FileSystem.h"
+#include "llvm/Support/FileSystem.h"
 
 #include "gtest/gtest.h"
 

flang/unittests/Frontend/FrontendActionTest.cpp

@@ -6,14 +6,15 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "gtest/gtest.h"
 #include "flang/Frontend/CompilerInstance.h"
 #include "flang/Frontend/CompilerInvocation.h"
 #include "flang/Frontend/FrontendOptions.h"
 #include "flang/FrontendTool/Utils.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 
+#include "gtest/gtest.h"
+
 using namespace Fortran::frontend;
 
 namespace {
@@ -79,6 +80,31 @@ class FrontendActionTest : public ::testing::Test {
   }
 };
 
+TEST_F(FrontendActionTest, TestInputOutput) {
+  // Populate the input file with the pre-defined input and flush it.
+  *(inputFileOs_) << "End Program arithmetic";
+  inputFileOs_.reset();
+
+  // Set-up the action kind.
+  compInst_.invocation().frontendOpts().programAction_ = InputOutputTest;
+
+  // Set-up the output stream. Using output buffer wrapped as an output
+  // stream, as opposed to an actual file (or a file descriptor).
+  llvm::SmallVector<char, 256> outputFileBuffer;
+  std::unique_ptr<llvm::raw_pwrite_stream> outputFileStream(
+      new llvm::raw_svector_ostream(outputFileBuffer));
+  compInst_.set_outputStream(std::move(outputFileStream));
+
+  // Execute the action.
+  bool success = ExecuteCompilerInvocation(&compInst_);
+
+  // Validate the expected output.
+  EXPECT_TRUE(success);
+  EXPECT_TRUE(!outputFileBuffer.empty());
+  EXPECT_TRUE(llvm::StringRef(outputFileBuffer.data())
+                  .startswith("End Program arithmetic"));
+}
+
 TEST_F(FrontendActionTest, PrintPreprocessedInput) {
   // Populate the input file with the pre-defined input and flush it.
   *(inputFileOs_) << "#ifdef NEW\n"