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.

0 files changed, 0 insertions, 0 deletions