[BUGFIX] Hint to the correct unit test runner

Running the test suite with the cli dispatcher
does not work as the bootstrap is executed twice.

Give a hint to the correct command instead
of letting it fatal.

Releases: 6.2
Resolves: #52787
Change-Id: Ic1def6ed40959128282ef2b6093066218012aa77
Reviewed-on: https://review.typo3.org/24720
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel