[lldb/Test] Don't print 'command invoked'

The different tools constructing dotest invocations (lit and
lldb-dotest) already print the command invocation so there's no need to
print it again in the dotest output.

My motivation for removing it is that it doesn't include the Python
interpreter and every time I accidentally copy it, the command fails
with an `ImportError`.

Differential revision: https://reviews.llvm.org/D81032