tests: tweak use of impacket in smbserver

tests: tweak use of impacket in smbserver

The missing message only uses a single line now and has a consistent
prefix making it easier for log parsers to skip. Remove a precheck test
looking for impacket in test1451 because it's incomplete, unnecessary
and in the wrong layer; the test harness will automatically discover
that the smbserver won't start and the test was only checking a single
dependency.  Also update the tests README to no longer mention Python 2.
The servers were switched to Python 3 several years ago and no attempt
is being made to maintain Python 2 compatibility

Closes #14669