Enable unit tests on ERF32 for /src/credentials/tests and /src/lib/support/tests

[feasel0]

Test issue(s)

The unit tests in /src/credentials/tests and /src/lib/support/tests are disabled when building for EFR32. They could be enabled if _open were implemented. Here is the error that occurs currently when attempting to build one of these tests.

[3452/4220] ld tests/matter-silabs-device_tests-TestBufferReader.out
FAILED: tests/matter-silabs-device_tests-TestBufferReader.out
arm-none-eabi-g++ -T/workspaces/connectedhomeip/src/test_driver/efr32/third_party/connectedhomeip/examples/platform/silabs/ldscripts/efr32mg24.ld -Wl,--no-warn-rwx-segment -march=armv8-m.main+dsp -mcpu=cortex-m33 -mabi=aapcs -mfpu=fpv5-sp-d16 -mfloat-abi=hard -mthumb -Os --specs=nosys.specs --specs=nano.specs -Werror -Wl,--fatal-warnings -fdiagnostics-color -Wl,--gc-sections @tests/matter-silabs-device_tests-TestBufferReader.out.rsp -o tests/matter-silabs-device_tests-TestBufferReader.out
/home/vscode/pigweed/env/cipd/packages/arm/bin/../lib/gcc/arm-none-eabi/12.2.1/../../../../arm-none-eabi/bin/ld: /home/vscode/pigweed/env/cipd/packages/arm/bin/../lib/gcc/arm-none-eabi/12.2.1/../../../../arm-none-eabi/lib/thumb/v8-m.main+fp/hard/libc_nano.a(libc_a-openr.o): in function '_open_r':
openr.c:(.text._open_r+0x10): warning: _open is not implemented and will always fail
collect2: error: ld returned 1 exit status

Platform

No response

Anything else?

No response