Fully rework the conformance test suite

The current conformance test suite is suboptimal in many ways.

All tests are built into the same binary, which makes adding new tests,
builting tests, and running groups of tests much more awkward than it
needs to be. The first issue, especially, raises the bar of contribution
in a significant way, while the other two take their toll on the
maintainer. All of these changes were introduced back when we had both
Clutter and Cogl tests in tree, and because we were building the test
suite for every single change; since then, Cogl moved out of tree with
all its tests, and we build the conformance test suite only when running
the `check` make target.

This admittedly large-ish commit changes the way the conformance test
suite works, taking advantage of the changes in the GTest API and test
harness.

First of all, all tests are now built separately, using their own test
suite as defined by each separate file. All tests run under the TAP
harness provided by GTest and Automake, to gather a proper report using
the Test Anything Protocol without using the `gtester` harness and the
`gtester-report` script. We also use the Makefile rules provided by GLib
to vastly simplify the build environment for the conformance test suite.

On top of the changes for the build and harness, we also provide new API
for creating and running test suites for Clutter. The API is public,
because the test suite has to use it, but it's minimal and mostly
provides convenience wrappers around GTest that make writing test units
for Clutter easier.

This commit disables all tests in the conformance test suite, as well as
moving the data files outside of the tests/data directory; the next few
commits will re-establish the conformance test suite separately so we
can check that everything works in a reliable way.
conformance: Add actor tests

Port the ClutterActor tests to the test API, and ensure they run under
the new TAP harness.
conformance: Add more tests

Add back some deprecated and general purpose API tests. These are the
ones that were written already pretty much conforming to the GTest API
and style, and thus require minimal porting.
tests: Use an internal setter for disabling vblank sync

Instead of using g_setenv().
build: Ignore *.test files

Generated when enabling installed tests.
Make abicheck.sh output TAP

So that we can run it under the TAP harness like the rest of the
conformance test suite.