mktemp: add test to . in template prefix

tests/by-util/test_mktemp.rs

@@ -21,6 +21,7 @@ static TEST_TEMPLATE6: &str = "tempXXXlate"; // spell-checker:disable-line
 static TEST_TEMPLATE7: &str = "XXXtemplate"; // spell-checker:disable-line
 #[cfg(unix)]
 static TEST_TEMPLATE8: &str = "tempXXXl/ate";
+static TEST_TEMPLATE9: &str = "a.XXXX";
 #[cfg(windows)]
 static TEST_TEMPLATE8: &str = "tempXXXl\\ate";
 
@@ -569,6 +570,14 @@ fn test_template_path_separator() {
         ));
 }
 
+/// Test that a prefix with a point is valid.
+#[test]
+fn test_prefix_template_separator() {
+    new_ucmd!()
+        .args(&["-t", TEST_TEMPLATE9])
+        .succeeds();
+}
+
 /// Test that a suffix with a path separator is invalid.
 #[test]
 fn test_suffix_path_separator() {