Enable running sql model tests in parallel

Each model test gets its own temporary directory, thus there is no problem parallelizing them.

This should reduce the time spent for a testrun of qtbase by around 3 minutes.
When running only the sql tests it saves less since there are no other tests to run in parallel.

=== Timing: =================== TEST RUN COMPLETED! ============================
  Total:                                       4 minutes 43 seconds
  Serial tests:                                4 minutes 42 seconds
  Parallel tests:                              1 second
  Estimated time spent on insignificant tests: (no time)
  Estimated time saved by -j4:                 1 second
=== Totals: 12 tests, 12 passes ================================================

After:

=== Timing: =================== TEST RUN COMPLETED! ============================
  Total:                                       4 minutes 7 seconds
  Serial tests:                                50 seconds
  Parallel tests:                              3 minutes 17 seconds
  Estimated time spent on insignificant tests: (no time)
  Estimated time saved by -j4:                 1 minute 23 seconds
=== Totals: 12 tests, 12 passes ================================================

Change-Id: I355ceed62fae852ad8cd442848e263438afb738e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>