t: run t5551 tests with both HTTP and HTTP/2

We have occasionally seen bugs that affect Git running only against an
HTTP/2 web server, not an HTTP one. For instance, b66c77a64e (http:
match headers case-insensitively when redacting, 2021-09-22). But since
we have no test coverage using HTTP/2, we only uncover these bugs in the
wild.

That commit gives a recipe for converting our Apache setup to support
HTTP/2, but:

  - it's not necessarily portable

  - we don't want to just test HTTP/2; we really want to do a variety of
    basic tests for _both_ protocols

This patch handles both problems by running a duplicate of t5551
(labeled as t5559 here) with an alternate-universe setup that enables
HTTP/2. So we'll continue to run t5551 as before, but run the same
battery of tests again with HTTP/2. If HTTP/2 isn't supported on a given
platform, then t5559 should bail during the webserver setup, and
gracefully skip all tests (unless GIT_TEST_HTTPD has been changed from
"auto" to "yes", where the point is to complain when webserver setup
fails).

In theory other http-related test scripts could benefit from the same
duplication, but doing t5551 should give us a reasonable check of basic
functionality, and would have caught both bugs we've seen in the wild
with HTTP/2.

A few notes on the implementation:

  - a script enables the server side config by calling enable_http2
    before starting the webserver. This avoids even trying to load any
    HTTP/2 config for t5551 (which is what lets it keep working with
    regular HTTP even on systems that don't support it). This also sets
    a prereq which can be used by individual tests.

  - As discussed in b66c77a64e, the http2 module isn't compatible with
    the "prefork" mpm, so we need to pick something else. I chose
    "event" here, which works on my Debian system, but it's possible
    there are platforms which would prefer something else. We can adjust
    that later if somebody finds such a platform.

  - The test "large fetch-pack requests can be sent using chunked
    encoding" makes sure we use a chunked transfer-encoding by looking
    for that header in the trace. But since HTTP/2 has its own streaming
    mechanisms, we won't find such a header. We could skip the test
    entirely by marking it with !HTTP2. But there's some value in making
    sure that the fetch itself succeeded. So instead, we'll confirm that
    either we're using HTTP2 _or_ we saw the expected chunked header.

  - the redaction tests fail under HTTP/2 with recent versions of curl.
    This is a bug! I've marked them with !HTTP2 here to skip them under
    t5559 for the moment. Using test_expect_failure would be more
    appropriate, but would require a bunch of boilerplate. Since we'll
    be fixing them momentarily, let's just skip them for now to keep the
    test suite bisectable, and we can re-enable them in the commit that
    fixes the bug.

  - one alternative layout would be to push most of t5551 into a
    lib-t5551.sh script, then source it from both t5551 and t5559.
    Keeping t5551 intact seemed a little simpler, as its one less level
    of indirection for people fixing bugs/regressions in the non-HTTP/2
    tests.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

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