Add gdb.ada/info_addr_mixed_case new testcase

This patch adds a new testcase to demonstrate a regression introduced by:

    commit b5ec771e
    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.