test_ipa_subdomains_server: Fix build with --coverage
    
    It seems that gcc did some optimization and
    used execve instead of execle when the code was
    instrumented for coverage analysis.
    
    So the exec* function was not wrapped and it tried to call
    real binary ipa-getkeytab
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>