tests: fight G++ warnings about zero as null pointer constant

In C++ pre C++11 it is standard practice to use 0 for the null pointer.
But GCC pre 8 -std=c++98 with -Wzero-as-null-pointer-constant warns about
this.

So disable -Wzero-as-null-pointer-constant when compiling C++ pre 11.
Let's do this in AT_DATA_SOURCE_PROLOGUE (which is pasted on top of
all the test grammar files).  Unfortunately, that shifts all the
locations in the expected error messages, which would be too noisy.
Instead, let's introduce testsuite.h, which can vary in length, and
include it in AT_DATA_SOURCE_PROLOGUE.

* tests/testsuite.h: New.
Disable -Wzero-as-null-pointer-constant's warning with GCC pre 8,
C++ pre 11.
* tests/local.at (AT_DATA_SOURCE_PROLOGUE): Use it.
* tests/atlocal.in (CPPFLAGS): Find it.
* tests/local.mk: Ship it.
* data/c.m4 (YY_NULLPTR): Prefer ((void*)0) to 0 in C.

tests/local.at

@@ -347,11 +347,11 @@ m4_define([AT_LANG_DISPATCH],
 # The prologue that should be included in any source code that is
 # meant to be compiled.  Keep atlocal.in sync (BISON_CXX_WORKS).
 m4_define([AT_DATA_SOURCE_PROLOGUE],
-[[#include <config.h>
-/* We don't need perfect functions for these tests. */
-#undef malloc
-#undef memcmp
-#undef realloc
+[[/* Load config.h, and adjust to the compiler.
+  We used to do it here, but each time we add a new line,
+  we have to adjust all the line numbers in error messages.
+  It's simpler to use a constant include to a varying file.  */
+#include <testsuite.h>
 ]])
 
 # AT_DATA_GRAMMAR_PROLOGUE