Testsuite: Provide '>>> ' for script input to allow binary data (take 2)

This mimics the '>>> ' prefix known for the test client. Any line prefixed
with '>>> ' will be processed by Perl's string eval().

As '>>> ' is generic and documented, it replaces the (undocumented)
'\NONL\' tag.

The client input lines starting with '>>> ' are now changed to '\>>> '
to avoid evaluation by the runtest script. (Test 4030, 1101).