test: Replace parser tests with empty framework

This change is a major rework of the parser tests. Currently, the parser
tests just run the parsers on each subdir of data/, but we don't
actually check the results.

Rather than just running parsers and expecting a certain fixed output,
the new tests are arbitraty (short!) C programs. This allows us to set
up the device configuration / discover context etc in whatever manner
necessary, and check that the boot options that the parsers emit are
correct.

This change removes all of the old tests, and adds a new "null" test.
Actual tests will be added in subsequent changes.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>