GoogleTest: Fix CTest not failing if gtest_discover_tests fails

Fixes regression introduced dac201442d (GoogleTest: Optimize gtest_discover_tests, 2020-02-18).

The generated CTest include files has the form:

    if(EXISTS "foo_tests.cmake")
      include("foo_tests.cmake")
    else()
      add_test(foo_NOT_BUILT foo_test_NOT_BUILT)
    endif()

Starting in dac201442d, an empty discovery_timeout_test[1]_tests.cmake was written
as soon as GoogleTestAddTests was processed.

This meant, that even if test discovery would fail (due to a crash or timeout in the executable),
we would always produce an empty CTest file.

So instead of reporting:
   Unable to find executable: foo_NOT_BUILT
   Errors while running CTest

We instead get:
   No tests were found!!!

To fix the problem, we WRITE the file on the first call to flush_script,
thus creating the file once we know we have valid output
and the call to gtest_discover_tests hasn't failed.

After creating the file, we then set the mode to APPEND
and append to the file for every subsequent call.

Modules/GoogleTestAddTests.cmake

@@ -13,11 +13,13 @@ set(tests)
 set(tests_buffer)
 
 # Overwrite possibly existing ${CTEST_FILE} with empty file
-file(WRITE "${CTEST_FILE}" "")
+set(flush_tests_MODE WRITE)
 
 # Flushes script to ${CTEST_FILE}
 macro(flush_script)
-  file(APPEND "${CTEST_FILE}" "${script}")
+  file(${flush_tests_MODE} "${CTEST_FILE}" "${script}")
+  set(flush_tests_MODE APPEND)
+
   set(script "")
 endmacro()