GDB/testsuite: Tighten `set print elements' error check

GDB/testsuite: Tighten `set print elements' error check

Match the whole error message expected to be given rather than omitting
the part about the "unlimited" keyword.  There's no point in omitting
the missing part first, and second with an upcoming change the part in
parentheses will no longer be a fixed string, so doing a full match will
ensure the algorithm correctly builds the message expected here.  Also
avoid any wildcard matches.