test: Make MBED_ASSERT failures fail

Make MBED_ASSERT failures fail the unit test case. Without this, we
might not notice the assertion failure as it wouldn't be bubbled up to
googletest.

platform/tests/UNITTESTS/doubles/CMakeLists.txt

@@ -35,4 +35,5 @@ target_link_libraries(mbed-stubs-platform
         mbed-headers-base
         mbed-headers-hal
         mbed-headers-platform
+        gmock_main
 )

platform/tests/UNITTESTS/doubles/mbed_assert_stub.cpp

@@ -16,6 +16,7 @@
  */
 
 #include "platform/mbed_assert.h"
+#include "gtest/gtest.h"
 #include <stdio.h>
 #include <stdbool.h>
 
@@ -27,4 +28,10 @@ extern "C" void mbed_assert_internal(const char *expr, const char *file, int lin
     if (mbed_assert_throw_errors) {
         throw 1;
     }
+
+    /* Ensure we fail the unit test if the Mbed assertion fails. Without this,
+     * we might not notice the assertion failure as it wouldn't be bubbled up
+     * to googletest. Note that this is after the above throw, as some tests
+     * check that an exception is thrown (i.e. negative tests). */
+    FAIL();
 }