tests: allow use of `prove' to run automake testsuite

tests: allow use of `prove' to run automake testsuite

* tests/prove-runner: New helper script; mostly it wraps
our `*.test' test scripts to make them runnable by the
`prove' utility.
* tests/Makefile.am (EXTRA_DIST): Distribute it.
(PROVE, AM_PROVEFLAGS, AM_PROVECMD): New variables.
(prove, installprove): New targets, used run the automake
testsuite through the `prove' utility.
* tests/gen-testsuite-part: For any considered dependency,
generate a variables that groups all the tests depending on
it.  This is especially useful for use in the new `prove'
and `installprove' targets, which cannot rely on make for
inter-test dependencies, and have to handle them "by hand".