Tests: Make test_compress.sh more flexible

Tests: Make test_compress.sh more flexible

Add a new optional second argument: directory of the xz and xzdec
executables. This is need with the CMake build where the binaries
end up in the top-level build directory.

If ../config.h doesn't exist, assume that all encoders and decoders
are available. This will make this script usable from CMake in the
most common build configuration.

NOTE: Since the existence of ../config.h is checked, the working
directory of the test script must be a subdir in the build tree!
Otherwise ../config.h would look outside the build tree.

Use the default check type instead of forcing CRC32 or CRC64.
Now the script doesn't need to check if CRC64 is available.