Makefile: do not put TESTS into check_PROGRAMS

If you assign TESTS into check_PROGRAMS, you cannot add a test that is
not built from source files.

Instead, use a temporary variable built_test_programs to hold the names
that are both programs built from source files and tests to execute.

This change is required by the following patch adding wayland-scanner
test script.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>