tests/gcov: demonstrate wrong coverage report after vfork/exec

Discovered in usr.bin/make, function Cmd_Exec.

The coverage test I ran on 2024-07-13 was still good. I don't remember
the exact version of NetBSD-current I was running back then.

With NetBSD-current from 2025-01-17, gcov does not report full coverage
data after a vfork/exec call. Running the test program inside ktrace
shows that after a vfork call, the child process writes its coverage data
back, probably right before the exec call, but the parent process
doesn't.

Running a child process through system(3) is not affected; there,
posix_spawn is used instead of vfork/exec.