tests: write tests/failures to build- instead of srcdir

All other test output goes to builddir too and moreover "make distcheck"
complains about writing to it's read-only srcdir.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: return error if failures file not usable

In past this could happen when doing make distcheck.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
build-sys: libmount/python/__init__.py is always a dist file

"make dist" and "make distcheck" should work after a bare
./configure to give us a full featured tar ball.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: try hard to create swaplabel's test image

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>