tests: initial port to the gtestutils framework

Some programs need to be split up into more tests, and the debug
output is mostly not updated for the new format.
tests: Add resources/* to soup-test.gresource, add utility functions

Add the files in resources/ to soup-test.gresource, and add
soup_test_load_resource() to get the contents of one of them, and then
use this in places that were previously reading them by hand.

Except forms-test, which needs to pass a filename to curl, so have it
use index.txt instead of one of the resource files.

Now none of the tests access the resources/ directory at runtime.

Also add soup_test_get_index() to read index.txt rather than
reimplementing it in multiple test programs.
tests: use TAP, and install the tests

Import glib-tap.mk to run the tests in TAP mode, and to provide rules
to install the tests.

This required some restructuring of httpd.conf-related stuff in order
to come up with a configuration that would work for all three cases
(srcdir==buildir, srcdir!=builddir, and installed).