build-sys: split test cases in lib/ into their own files

In order to avoid compiling the same source files twice, with and
without the TEST_PROGRAM define.

Tested that the build still works and that `make distcheck` works as
expected.

Tested that the test_* programs in lib/ keep working. (Though they are
not really invoked by `make check` and in particular test_nsutils is
quite useless, test_fileutils also quite poor.)

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

lib/test_fileutils.c

@@ -0,0 +1,10 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include "fileutils.h"
+
+int main(int argc, char *argv[])
+{
+	atexit(close_stdout);
+	printf("Hello, World!\n");
+	return EXIT_SUCCESS;
+}