structinit test: only run on DEBUG builds

The structinit tests don't make sense unless structure padding
is uniformly initialized, which is unlikely to happen on release
builds.  Only enable them for DEBUG builds.  Further, rename them
to core::structinit.

tests/structinit/structinit.c → tests/core/structinit.c

@@ -24,6 +24,7 @@ do { \
 	structname structname##_macro_latest = macroinit; \
 	structname structname##_func_latest; \
 	int structname##_curr_ver = structver - 1; \
+	memset(&structname##_func_latest, 0, sizeof(structname##_func_latest)); \
 	cl_git_pass(funcinitname(&structname##_func_latest, structver)); \
 	options_cmp(&structname##_macro_latest, &structname##_func_latest, \
 		sizeof(structname), STRINGIFY(structname)); \
@@ -54,8 +55,22 @@ static void options_cmp(void *one, void *two, size_t size, const char *name)
 	}
 }
 
-void test_structinit_structinit__compare(void)
+void test_core_structinit__compare(void)
 {
+	/* These tests assume that they can memcmp() two structures that were
+	 * initialized with the same static initializer.  Eg,
+	 * git_blame_options = GIT_BLAME_OPTIONS_INIT;
+	 *
+	 * This assumption fails when there is padding between structure members,
+	 * which is not guaranteed to be initialized to anything sane at all.
+	 *
+	 * Assume most compilers, in a debug build, will clear that memory for
+	 * us or set it to sentinal markers.  Etc.
+	 */
+#if !defined(DEBUG) && !defined(_DEBUG)
+	clar__skip();
+#endif
+
 	/* blame */
 	CHECK_MACRO_FUNC_INIT_EQUAL( \
 		git_blame_options, GIT_BLAME_OPTIONS_VERSION, \