run-tests: cache hghave results Spawning a process on Windows is expensive. I've got a version of test-lfs-test-server.t locally which prints the http request/responses that totals 819 lines, with 149 conditional lines, 11 #if tests, and 2 test cases. It takes just under 1 minute with this change to run both cases, vs just over 2 minutes without this change. Worse, when I explored adding ui.debug to the test, it takes 13 minutes due to all of the mismatches and retests, vs less than 1 minute with this change. Overall, the difference when running all tests is negligible- 103 minutes with this change, vs 105 without when using -j9. It also looks like an exit value of 2 from `hghave` is treated specially, but there's nothing preventing 2 missing features from also using this value.