[gdb/testsuite] Add gdb.dwarf2/debug-names-tu.exp

Add a test-case gdb.dwarf2/debug-names-tu.exp, that uses the dwarf assembler
to specify a .debug_names index with the TU list referring to a TU from the
.debug_types section.

This is intended to produce something similar to:
...
$ gcc -g -fdebug-types-section ~/hello.c -gdwarf-4
$ gdb-add-index -dwarf-5 a.out
...

Tested on x86_64-linux.