Enhance tests to work on different systems

Enhance tests to work on different systems

The GNU platform testers reported a number of test errors on
different systems; try to address them.

* tests/thelp.pl: A number of tests timed out with a 4-second
timeout.  Increase the default timeout to 10 seconds.
* tests/run_make_tests.pl: Executing directories on cygwin behaves
differently in Perl than make so skip these tests there.
* tests/scripts/options/symlinks: Check for the symlink feature
in make, rather than whether the system supports them.
* tests/scripts/features/implicit_search: On some systems "false"
exits with a different exit code.  Use the helper instead.
* tests/scripts/features/loadapi: Ditto.
* tests/scripts/features/output-sync: Sleep before make -f bar in
the first test as well as the second one.
* tests/scripts/features/exec: Skip on cygwin, which seems to
be "UNIX" but where scripts don't run normally.
* tests/scripts/misc/fopen-fail: Skip on cygwin, where make
eventually exits with exit code 0 and no error messages.