Before this change the tests ran in about 12m34 and afterwards in
about 6m20, although the real change is more dramatic if you only run
tests from the tests/ subdirectory (as language tests still run serially).
This breaks valgrinding for now, which I intend to fix properly later.
Apply this change across all the shell scripts containing tests.
Additionally this defines the environment variables $abs_srcdir,
$abs_builddir, $top_srcdir, $top_builddir, $abs_top_srcdir and
$abs_top_builddir which can now be used throughout test scripts.
All tests run under the ./run binary. For a long time the ./run
binary has set the $PATH environment variable to contain all of the
directories with binaries in them.
Therefore there is no reason to use ../fish/guestfish instead of just
plain guestfish (and the same applies to other built binaries).
