t5559: make SSL/TLS the default

t5559: make SSL/TLS the default

The point of t5559 is run the regular t5551 tests with HTTP/2. But it
does so with the "h2c" protocol, which uses cleartext upgrades from
HTTP/1.1 to HTTP/2 (rather than learning about HTTP/2 support during the
TLS negotiation).

This has a few problems:

 - it's not very indicative of the real world. In practice, most servers
   that support HTTP/2 will also support TLS.

 - support for upgrading does not seem as robust. In particular, we've
   run into bugs in some versions of Apache's mod_http2 that trigger
   only with the upgrade mode. See:

     https://lore.kernel.org/git/Y8ztIqYgVCPILJlO@coredump.intra.peff.net/

So the upside is that this change makes our HTTP/2 tests more robust and
more realistic. The downside is that if we can't set up SSL for any
reason, we'll skip the tests (even though you _might_ have been able to
run the HTTP/2 tests the old way). We could probably have a conditional
fallback, but it would be complicated for little gain, and it's not even
clear it would help (i.e., would any test environment even have HTTP/2
but not SSL support?).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>