Overhaul testing library constructors

We want to make it easier to test a bunch of different functions that
might be invoked in constructors of other libraries.

It seems conceivable that with these snippets, we could design other
tests that also work across a wide range of intercepted functions.

test/_libtest.c

@@ -0,0 +1,8 @@
+#include "snippets/include_headers.h"
+#define where "library"
+void FUNC_NAME_as_needed() {
+  printf("  called FUNC_NAME_as_needed() \n");
+}
+static __attribute__((constructor)) void init_FUNC_NAME() {
+#include "snippets/FUNC_NAME.c"
+}