[gdb/testsuite] Fix duplicate test-names in gdb.multi

[gdb/testsuite] Fix duplicate test-names in gdb.multi

In gdb.multi we have:
...
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=run: inf2_how=run: info inferiors
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=run: inf2_how=attach: info inferiors
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=run: inf2_how=tty: info inferiors
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=attach: inf2_how=run: info inferiors
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=attach: inf2_how=attach: attach
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=attach: inf2_how=attach: info inferiors
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=attach: inf2_how=tty: info inferiors
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=tty: inf2_how=run: info inferiors
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=tty: inf2_how=attach: info inferiors
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=tty: inf2_how=tty: tty TTY
DUPLICATE: gdb.multi/multi-term-settings.exp: \
  inf1_how=tty: inf2_how=tty: info inferiors
...

Fix these using with_test_prefix.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-12  Tom de Vries  <tdevries@suse.de>

	* gdb.multi/multi-term-settings.exp: Use with_test_prefix.