gdb/testsuite: extend special '^' handling to gdb_test_multiple

gdb/testsuite: extend special '^' handling to gdb_test_multiple

The commit:

  commit 08ec06d6440745ef9204d39197aa1e732df41056
  Date:   Wed Mar 29 10:41:07 2023 +0100

      gdb/testsuite: special case '^' in gdb_test pattern

Added some special handling of '^' to gdb_test -- a leading '^' will
cause the command regexp to automatically be included in the expected
output pattern.

It was pointed out that the '-wrap' flag of gdb_test_multiple is
supposed to work in the same way as gdb_test, and that the recent
changes for '^' had not been replicated for gdb_test_multiple.  This
patch addresses this issue.

So, after this commit, the following two constructs should have the
same meaning:

  gdb_test "command" "^output" "test name"

  gdb_test_multiple "command" "test name" {
    -re -wrap "^output" {
      pass $gdb_test_name
    }
  }

In both cases the '^' will case gdb.exp to inject a regexp that
matches 'command' after the '^' and before the 'output', this is in
addition to adding the $gdb_prompt pattern after 'output' in the
normal way.

The special '^' handling is only applied when '-wrap' is used, as this
is the only mode that aims to mimic gdb_test.

While working on this patch I realised that I could actually improve
the logic for the special '^' handling in the case where the expected
output pattern is empty.  I replicated these updates for both gdb_test
and gdb_test_multiple in order to keep these two paths in sync.

There were a small number of tests that needed adjustment after this
change, mostly just removing command regexps that are now added
automatically, but the gdb.base/settings.exp case was a little weird
as it turns out trying to match a single blank line is probably harder
now than it used to be -- still, I suspect this is a pretty rare case,
so I think the benefits (improved anchoring) outweigh this small
downside (IMHO).