validate:launcher: Do not CK_FORK on our test

Otherwise the process can't cleanly quit on assertion because of the way libcheck runner is implemented
4 jobs for gdb_ourself in 8 minutes (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #58600
gst indent

00:00:19

passed #58599
manifest

00:00:24

 
  Build
passed #58601
build fedora x86_64

00:03:06

 
  Full Builds
passed #58602
build android arm64 api28

00:04:30