Add gdb.ada/info_addr_mixed_case new testcase
    
    This patch adds a new testcase to demonstrate a regression introduced by:
    
        commit b5ec771e60c1a0863e51eb491c85c674097e9e13
        Date:   Wed Nov 8 14:22:32 2017 +0000
        Subject: Introduce lookup_name_info and generalize Ada's FULL/WILD name matching
    
    The purpose of the testcase is to verify that a user can use any
    casing for an Ada symbol name passed to the "info address" command.
    After the patch above was applied, GDB was no longer able to find
    the symbol:
    
        (gdb) info address My_Table
        No symbol "My_Table" in current context.
    
    gdb/testsuite/ChangeLog:
    
    	PR gdb/22670
    	* gdb.ada/info_addr_mixed_case: New testcase.
    
    Tested on x86_64-linux, both before and after the patch.