gdb/testsuite: avoid {"} pattern in lib/mi-support.exp

gdb/testsuite: avoid {"} pattern in lib/mi-support.exp

Commit:

  commit c569a946f6925d3f210c3eaf74dcda56843350ef
  Date:   Fri Mar 24 10:45:37 2023 +0100

      [gdb/testsuite] Fix unbalanced quotes in mi_expect_stop argument

Introduced the use of {"} in mi-support.exp.  There is absolutely
nothing wrong with this in any way.  However, this is causing my
editor to get the syntax highlighting of this file wrong after this
point.

Maybe the real answer is to use a better editor, or fix my current
editor.... but I'm hoping I can instead take the lazy approach of just
changing {"} to "\"", which is handled fine, and means exactly the
same as far as I understand it.

There should be no change in what is tested after this commit.

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