Avoid blank lines in backslash warnings test.

This is a follow-up to 12f55bbce575aecc, which fell victim to a
bug workaround. Because the broken pipes on VMS sometimes put
multiple blank lines in test output, we collapse multiple newlines
into one.  Which breaks tests that genuinely have multiple blank
lines in the output.

So don't leave the output blank, which coincidentally makes it a
lot easier to see which print statement produces which line of
output.