tests/fmt_print: spice up test

Add also a test for correct behavior of `print()` when the string
doesn't happen to be zero-terminated at the expected position.

tests/fmt_print/main.c

@@ -45,7 +45,11 @@ int main(void)
     print_str("\n");
     print_float(1.2345, 5);
     print_str("\n");
-    printf("%s", "Test ");
+    /* test mixing of printf() and fmt's print() works fine */
+    printf("%s", "Test");
+    /* test fmt's print indeed only honors the length parameter and doesn't
+     * print until the terminated zero byte */
+    print(" not ", 1);
     print_str("successful.");
     puts("");