bug#44458: Regenerating testsuite-part.am when a new test case is added

[Karl Berry]

    without depending on list-of-tests.mk, the dependencies of new tests
    are not extracted automatically, and so tests that depend on other
    tests can be run in the wrong order

Indeed. For the record (as Reuben already knows :), I was seeing the
same unpredictable behavior wrt the new vala tests (in #13002) not
finding pkg-config.

Thus I have pushed the following change to try to revert that part of
the #11347 change. Fingers crossed.

Thanks much. -k

testsuite-dependencies.diff
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