[gdb/testsuite] Fix -wrap in presence of -prompt in gdb_test_multiple

[gdb/testsuite] Fix -wrap in presence of -prompt in gdb_test_multiple

While writing a gdb_test_multiple call in a test-case I tried to use -wrap in
combination with -prompt and found out that it doesn't work, because -wrap uses
"$gdb_prompt $" instead of $prompt_regexp.

Fix this by making -wrap use $prompt_regexp.

Tested on x86_64-linux.