tests: Avoid backslashes in awk variable assignment.

tests: Avoid backslashes in awk variable assignment.

Backslashes in awk command-line variable assignments are normally
interpreted as beginning escape sequences, which is not what we
want to do for the error message prefix test.

Just exchanging backslashes with some other character (let's pick
forward slashes) should be more than sufficient for this test, to
ensure that the program name prefixes error messages.