[tests] lib/libc/gen:assert_test: skip tests when NDEBUG set

In the event that NDEBUG is specified in CFLAGS--which is most likely
triggered via `MK_ASSERT_DEBUG` == "no" -- all assert(3) statements are
optimized out by design. This breaks the assert(3) tests as they
specifically rely on assert(3) actually raising a `SIGABRT` instead of
quietly succeeding.

Skip both tests if `NDEBUG` is set. There's no sense running the
`assert(true)` test if the result could instead be a false positive.

MFC after:	2 weeks

contrib/netbsd-tests/lib/libc/gen/t_assert.c

@@ -43,6 +43,18 @@ __RCSID("$NetBSD: t_assert.c,v 1.3 2017/01/10 15:17:57 christos Exp $");
 #include <string.h>
 #include <unistd.h>
 
+#ifdef	__FreeBSD__
+/* TODO: upstream this. */
+static void
+skip_if_ndebug_set(void)
+{
+
+#ifdef	NDEBUG
+	atf_tc_skip("Testcase requires code to be compiled without -DNDEBUG.");
+#endif
+}
+#endif
+
 static void
 disable_corefile(void)
 {
@@ -74,6 +86,10 @@ ATF_TC_BODY(assert_false, tc)
 	pid_t pid;
 	int sta;
 
+#ifdef	__FreeBSD__
+	skip_if_ndebug_set();
+#endif
+
 	pid = fork();
 	ATF_REQUIRE(pid >= 0);
 
@@ -112,6 +128,10 @@ ATF_TC_BODY(assert_true, tc)
 	pid_t pid;
 	int sta;
 
+#ifdef	__FreeBSD__
+	skip_if_ndebug_set();
+#endif
+
 	pid = fork();
 	ATF_REQUIRE(pid >= 0);