Add quotation mark in test message

Add quotation mark in test message

I happen to see a quotation mark is missing the following test,

 gdb_test "break $end_location" \
     "Breakpoint $decimal at .* line $end_location\." \
     set breakpoint at end of main"

so the test result is

PASS: gdb.reverse/break-reverse.exp: set

This patch is to add the missing quotation mark back, and the test
result becomes

PASS: gdb.reverse/break-reverse.exp: set breakpoint at end of main

gdb/testsuite:

2016-03-24  Yao Qi  <yao.qi@linaro.org>

	* gdb.reverse/break-reverse.exp: Add quotation mark in the
	test message.