configure.ac: belatedly bump glib requirement to 2.38

The tests use the TAP driver now, which only exists in glib 2.38 and
later.
test-utils: change http_debug command-line flag

g_test_init() intercepts "-h" for "help", so make http-debug be "-H"
xmlrpc-test: use g_test_skip()

There was a comment about not using g_test_skip() here because of an
old glib bug, but we're using g_test_skip() in lots of other places
now, so...
redirect-test: remove a flaky test

Bug 651446 involved a race condition when queuing a message while
another message was being redirected on the same host. The fix
included a regression test, but later rewrites of SoupSession made the
race condition no longer possible, and made the test itself slightly
flaky. So just remove it.
auth-test: remove some erroneous SKIP_IF_NO_APACHEs
tests: split up some test programs into more tests
tests: remove debug_printf()s that are redundant with test names
tests: use g_test_bug() to annotate test cases