Run tests for non-truecolor and non-egc builds.

Run tests for non-truecolor and non-egc builds.

This patch introduces a few new things:

* Build artifact termbox.ffi.h for easier FFI header file parsing. As a result, no more `__ffi_strip` bs, but probably more complex overall. The other idea I had would have been uglier. I think the test coverage is worth it.

* Functions `tb_has_truecolor` and `tb_has_egc` for determining support for those features at runtime.

* Test container uses `debian:11-slim` image instead of `debian:10-slim`.

* Test container uses PHP 8 instead of PHP 7.

* Test container now accepts `cflags` build argument.

* Tests can now be skipped via `$test->skip()`.

* Test suite covers 3 different builds: normal, non-truecolor, non-egc.