news: document improvement in parallel-tests performance

news: document improvement in parallel-tests performance

* NEWS: Document the improved performances of the parallel-tests targets
'check' and 'recheck'. See recent commits:

  - v1.12-20-gf5733aa of 2012-04-26,
    "parallel-tests: optimize 'recheck' target for speed"

  - v1.12-21-g5eeb366 of 2012-02-26,
    "parallel-tests: optimize global log creation"

  - v1.12-22-g4c2dd6e of 2012-04-30,
    "parallel-tests: save few forks when possible"

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>