[gdb/testsuite] Fix gdb.go/methods.exp with check-readmore

[gdb/testsuite] Fix gdb.go/methods.exp with check-readmore

When running test-case gdb.go/methods.exp with make check we have:
...
(gdb) break main.T.Foo^M
Function "main.T.Foo" not defined.^M
Make breakpoint pending on future shared library load? (y or [n]) n^M
(gdb) XFAIL: gdb.go/methods.exp: gdb_breakpoint: set breakpoint at main.T.Foo
...
but with make check-readmore the XFAIL fails to trigger:
...
(gdb) break main.T.Foo^M
Function "main.T.Foo" not defined.^M
Make breakpoint pending on future shared library load? (y or [n]) n^M
(gdb) FAIL: gdb.go/methods.exp: gdb_breakpoint: set breakpoint at main.T.Foo
...

This happens because this gdb_test_multiple "maintenance print symbols"
regexp:
...
    -re "\r\n$gdb_prompt $" {
...
matches the entire command output.

Fix this by adding the missing ^ at the regexp start.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29064