Let the mandoc.db(5) test suite work with parallel make (make -j),

Let the mandoc.db(5) test suite work with parallel make (make -j),
by making sure that different tests use different directory names
for their work such that they do not collide.
As a side benefit, this allows keeping the complete working directories
of the tests until "make cleandir" is run, which may occasionally
help debugging when something breaks.

The failure with make -j was detected and reported by anton@,
who also reviewed, tested, and OK'ed this somewhat lengthy patch.