Improve -Wunused-value testcase build failures fix

This commit improves upon my previous -Wunused-value fix by
replacing the various dummy variables with casts to void, as
suggested by Pedro.

gdb/testsuite/ChangeLog:

	* gdb.cp/namespace.cc: Improve -Wunused-value fix.
	* gdb.cp/nsimport.cc: Likewise.
	* gdb.cp/nsnested.cc: Likewise.
	* gdb.cp/nsnoimports.cc: Likewise.
	* gdb.cp/nsusing.cc: Likewise.
	* gdb.cp/smartp.cc: Likewise.
	* gdb.python/py-pp-integral.c: Likewise.
	* gdb.python/py-pp-re-notag.c: Likewise.