gtestutils: pass open file descriptors to subprocess

The subprocess needs to access the test_log_fd.  If the file descriptors
are not left open, functions such as g_test_message may stomp on file
descriptors open by the subprocess and cause bad behavior of the test.

(Tweaked by Philip Withnall <bugzilla@tecnocode.co.uk> to fix review
comments.)
7 jobs for 501-test-subprocess-fds in 13 minutes and 43 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #190971
cross-android_api21_arm64

00:02:21

passed #190972
cross-android_api28_arm64

00:02:21

passed #190973
cross-mingw64

00:02:42

passed #190970
fedora-x86_64

00:05:31

passed #190974
win32
msys2-mingw32

00:11:45

passed #190975
win32
vs2017-x64

00:08:22

 
  Coverage
passed #190976
coverage

00:01:57

74.2%