[gdb/testsuite] Fix control-flow in gdb.reverse/insn-reverse.exp

[gdb/testsuite] Fix control-flow in gdb.reverse/insn-reverse.exp

In gdb.reverse/insn-reverse.exp, we have loop containing a call to
gdb_test_multiple, which itself contains a break:
...
  for {} {$count < 500} {incr count} {
    ...
    gdb_test_multiple "x/i \$pc" "" {
      ...
      break
    }
...

On SLE-11 with:
...
$ runtest --version
Expect version is       5.44.1.11
Tcl version is          8.5
Framework version is    1.4.4
...
the break doesn't seem to have the effect of breaking out of the loop.

The break does have the effect of terminating evaluation of the expect clause,
which means we don't set insn_array, after which we run into:
...
ERROR: tcl error sourcing src/gdb/testsuite/gdb.reverse/insn-reverse.exp.
ERROR: can't read "insn_array(5)": no such element in array
...

gdb/testsuite/ChangeLog:

2020-12-04  Tom de Vries  <tdevries@suse.de>

	* gdb.reverse/insn-reverse.exp: Don't break inside gdb_test_multiple
	clause.