[gdb/testsuite] Don't use string_to_regexp twice in gdb.base/options.exp

[gdb/testsuite] Don't use string_to_regexp twice in gdb.base/options.exp

In test-case gdb.base/options.exp, in proc test_completer_recognizes we have:
...
    set expected_re [string_to_regexp $input_line]
    test_gdb_complete_unique $input_line $expected_re
...

However, the first thing we do in proc test_gdb_complete_unique	is to apply
string_to_regexp to the second argument:
...
proc test_gdb_complete_unique {
  input_line
  complete_line
  {append_char " "}
  {max_completions false}
  {testname ""}
} {
    set complete_line_re [string_to_regexp $complete_line]
    test_gdb_complete_unique_re \
         $input_line \
	 $complete_line_re \
	 $append_char \
	 $max_completions\
	 $testname
}
...

This happens to not cause any FAILs at the moment, but this should be done
only once.

Fix this not using string_to_regexp in proc test_completer_recognizes.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>