Enable expensive tests

This increases the test count from 11623 to 11929.
And running time (Core [email protected]) from 18 minutes to 24 minutes with
10 parallel jobs.

Signed-off-by: Thomas Braun <[email protected]>

Author: t-b

t/test-lib.sh

@@ -873,6 +873,7 @@ case $(uname -s) in
 	# no POSIX permissions
 	# backslashes in pathspec are converted to '/'
 	# exec does not inherit the PID
+	test_set_prereq EXPENSIVE
 	test_set_prereq MINGW
 	test_set_prereq NOT_CYGWIN
 	test_set_prereq NATIVE_CRLF