checks: move files required by unit tests into tests/files and make sure they're...

checks: move files required by unit tests into tests/files and make sure they're disted

Move unit test data into the directory where it belongs and make in particular
the flacdec unit test cd into the directory with the test files instead of making
assumptions about the current working directory in that unit test. As a side effect
of movng those files, there's only one EXTRA_DIST in tests/check/Makefile.am now,
which is likely to work better than having two. Hopefully fixes #582753.