[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.