gdb/testsuite: fix breakpoint script output in gdb.mi/mi-break.exp

gdb/testsuite: fix breakpoint script output in gdb.mi/mi-break.exp

Commit 9db0d8536dbc ("gdb/mi: fix breakpoint script field output") fixed
the output of the script key in the MI breakpoint output, from

  script={"print 10","continue"}

to

  script=["print 10","continue"]

However, it missed updating this test case, which still tests for the
old (broken) form, causing:

    FAIL: gdb.mi/mi-break.exp: mi-mode=main: test_breakpoint_commands: breakpoint commands: check that commands are set (unexpected output)
    FAIL: gdb.mi/mi-break.exp: mi-mode=separate: test_breakpoint_commands: breakpoint commands: check that commands are set (unexpected output)

Update the test to expect the new form.

Change-Id: I174919d4eea53e96d914ca9bd1cf6f01c8de30b8