[gdb/testsuite] Don't use FOOBAR pattern in gdb_test

[gdb/testsuite] Don't use FOOBAR pattern in gdb_test

If gdb_test is used with fewer than five arguments, then the question_string
defaults to "^FOOBAR$":
...
    if [llength $args]==5 {
       set question_string [lindex $args 3]
       set response_string [lindex $args 4]
    } else {
       set question_string "^FOOBAR$"
    }
...

This can however match "FOOBAR", so perhaps "\$FOOBAR^" would have been a
better choice.

Eliminate the FOOBAR pattern from gdb_test by instead of defining a default
regexp, conditionally appending the regexp matching to a user_code variable.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-09-19  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_test): Eliminate "^FOOBAR$" pattern.