test: Rename TEST_OUTPUT to COMPILE_OUTPUT

The real meaning for TEST_OUTPUT is to provide the correct output from the
compiler, not the whole test. This is specially confusing for runnable
tests where someone could think this test parameter is used to check
against the program output. For this reason the test parameter is renamed
to COMPILE_OUTPUT (which is the same internal name d_do_test.d use).

Also the documentation in test/Makefile is updated to describe the
previously missing test parameter.