Disable warnings in string/tester.c at top level.

string/tester.c contains code that correctly triggers various GCC
warnings about dubious uses of string functions (uses that are being
deliberately tested there), and duly disables those warnings around
the relevant code.

A change in GCC mainline resulted in this code failing to compile with
a -Warray-bounds error, despite the location with the error having
-Warray-bounds already disabled.  This has been reported as
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91890>.  This patch
avoids that problem and possible future issues with these diagnostics
by moving all the warning disabling in this file to top level, as
suggested by Florian in
<https://sourceware.org/ml/libc-alpha/2019-10/msg00033.html>, rather
than only doing it locally around specific function calls.

Tested with build-many-glibcs.py for aarch64-linux-gnu with GCC
mainline (with only the conform/ failures noted in
<https://sourceware.org/ml/libc-alpha/2019-10/msg00043.html>).

	* string/tester.c: Ignore -Warray-bounds and
	-Wmemset-transposed-args at top level.
	[__GNUC_PREREQ (7, 0)]: Ignore -Wrestrict and -Wstringop-overflow=
	at top level.
	[__GNUC_PREREQ (8, 0)]: Ignore -Wstringop-truncation at top level.
	(test_stpncpy): Do not ignore warnings here.
	(test_strncat): Likewise.
	(test_strncpy): Likewise.
	(test_memset): Likewise.