First attempt at sanitising testharness.

This is mostly to deal with out-of-tree issues:
* testharness no longer has any fluidity specific crap in it
* you can still ask for it to print the output of `which fluidity` and `fluidity --version` by giving the option --give-version=fluidity
* for out-of-tree builds we rely on testharness to be called as <build-dir>/bin/testharness ... As before testharness goes to its parent
directory and adds bin/, lib/ and python/ to PATH, LD_LIBRARY_PATH and PYTHONPATH.
*This should set the correct path for all executables run in the tests (fluidity, flredecomp, etc. etc.), so we no longer want ../../bin/ in any
of the .xml files (as this is incorrect for the out-of-tree situation where srcdir/tests/<test-dir>/../../bin leads to srcdir/bin which doesn't
exist
* After stripping the ../../bin/ from the tests we therefore no longer need to symlink <srcdir>/bin to <build-dir>/bin
Also strip ../../bin from Makefiles.
Bug-fixes after testharness sanitisation for out-of-tree.
More thoroughly strip ../../bin/ from tests.

Should have used s@../../bin@@g instead of s@../../bin@@