The chmod for phpunit binaries is not needed after ext:phpunit
fixed this, the execute script itself is simplified a bit to
be better readable.

Change-Id: I9b6d1d1e2e9e4155ee20c3dfda4313d89fed0def
Resolves: #51153
Releases: 6.2
Reviewed-on: https://review.typo3.org/23172
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn