[gdb/testsuite] Add proc require in lib/gdb.exp

Add a new proc require in lib/gdb.exp, and use it to shorten:
...
if { [gdb_skip_xml_test] } {
    # Valgrind gdbserver requires gdb with xml support.
    untested "missing xml support"
    return 0
}
...
into:
...
require gdb_skip_xml_test 0
...

Tested on x86_64-linux, both with and without a trigger patch that forces
gdb_skip_xml_test to return 1.