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.

tests/scripts/options/symlinks

@@ -6,14 +6,7 @@ $details = "Verify that symlink handling with and without -L works properly.";
 
 # Only run these tests if the system sypports symlinks
 
-# Apparently the Windows port of Perl reports that it does support symlinks
-# (in that the symlink() function doesn't fail) but it really doesn't, so
-# check for it explicitly.
-
-if ($port_type eq 'W32' || !( eval { symlink("",""); 1 })) {
-  # This test is N/A
-  return -1;
-}
+exists $FEATURES{'check-symlink'} or return -1;
 
 use File::Spec;