build-sys: fix some important 'tests.h' related issues

Exploiting a header file shouldn't also force users to
code their own '#include <stdio.h>'. More importantly,
unless this header is mentioned in dist_noinst_HEADERS
we'd fail 'make distcheck' with the error shown below.

[ the same error will apply to all 'noinst_PROGRAMS' ]
[ that happen to use this header. but please, do not ]
[ ask me to explain exactly why or how my fix works! ]

Reference(s):
../../proc/test_pids.c:24:10: fatal error: tests.h: No such file or directory
   24 | #include "tests.h"
      |          ^~~~~~~~~

Signed-off-by: Jim Warner <james.warner@comcast.net>

include/Makefile.am

@@ -6,4 +6,5 @@ dist_noinst_HEADERS = \
 	rpmatch.h \
 	signals.h \
 	strutils.h \
+	tests.h \
 	xalloc.h

include/tests.h

@@ -2,6 +2,8 @@
 #ifndef PROCPS_NG_TESTS_H
 #define PROCPS_NG_TESTS_H
 
+#include <stdio.h>
+
 typedef int (*TestFunction)(void *data);
 
 char *testname;