regres.cmd: print diff info only for verbose levels >2

 When running 'regress.cmd' for a couple of tests,
verbosity level 1 or even better 2 is necessary to to observe the progress of the test.

At the same time, in case of errors, the difference output is printed for verbosity level>0
That output makes it hard to follow the test progress immediately and distinguish failed tests from succeeded ones .

Proposal:

print diff output only for verbosity level >2