tdb2: speed up testing.

tdb2: speed up testing.

The time to run tests under valgrind has become excessive; particularly
the failure tests which fork().   Thus we cut down testing:

1) api-94-repack: reduce from 234 seconds to 2 seconds by cutting
   iterations, despite adding TDB_VERSION1 tests.

2) api-missing-entries: reduce from 17 seconds to under 1 second by
   not checking db inside loop, but at end.

This reduces the total ccanlint time from 729 to 489 seconds.