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/fileutils.c

@@ -43,13 +43,3 @@ void close_stdout(void)
 	if (close_stream(stderr) != 0)
 		_exit(EXIT_FAILURE);
 }
-
-#ifdef TEST_PROGRAM
-#include <stdio.h>
-int main(int argc, char *argv[])
-{
-	atexit(close_stdout);
-	printf("Hello, World!\n");
-	return EXIT_SUCCESS;
-}
-#endif				/* TEST_PROGRAM */