test: switch the unit tests to something resembling a test suite

The tests have inadvertent dependencies on each other so let's avoid
those by changing to a system that returns a null-terminated list of
test functions and our test runner iterates over those and forks off one
process per function.

test/xfree86.c

@@ -108,11 +108,13 @@ xfree86_add_comment(void)
     free(current);
 }
 
-int
+const testfunc_t*
 xfree86_test(void)
 {
-    xfree86_option_list_duplicate();
-    xfree86_add_comment();
-
-    return 0;
+    static const testfunc_t testfuncs[] = {
+        xfree86_option_list_duplicate,
+        xfree86_add_comment,
+        NULL,
+    };
+    return testfuncs;
 }