Remove extra output directory level for Ada tests

The output of Ada tests create a layout where the test name
("formatted_ref" in this example) appears twice:

outputs
└── gdb.ada
    └── formatted_ref
        └── formatted_ref
            ├── b~formatted_ref.adb
            ├── b~formatted_ref.ads
            ├── b~formatted_ref.ali
            ├── b~formatted_ref.o
            ├── defs.ali
            ├── defs.o
            ├── formatted_ref
            ├── formatted_ref.ali
            └── formatted_ref.o

This causes a problem when testing with the native-gdbserver board, when
the binary has the same name as the test.  When gdb_remote_download is
called to upload the compiled binary, the implementation for
native-gdbserver copies it in the standard output directory (in
outputs/gdb.ada/formatted_ref).  However, there is already a directory
named formatted_ref in there, so the copy fails and gdbserver isn't able
to load the binary.

This patch bypasses the problem by removing the extra directory level.
The compiled binary will already be in its final location in the
standard output directory, so the copy will effectively be a no-op.

gdb/testsuite/ChangeLog:

	* lib/ada.exp: Remove extra directory level in build directory.
	* gdb.ada/cond_lang.exp: Likewise.
	* gdb.ada/exec_changed.exp: Likewise.
	* gdb.ada/lang_switch.exp: Likewise.