Only test locked property file is not writeable.

src/test/kotlin/net/thauvin/erik/gradle/semver/UtilsSpec.kt

@@ -232,9 +232,11 @@ object UtilsSpec : Spek(
                 }
 
                 Then("saving the locked properties file") {
+                    if (!propsLocked.canWrite()) {
                        assertFailsWith<GradleException> {
                             Utils.saveProperties(propsLocked.parentFile, config, version)
                         }
+                    }
                     propsLocked.delete()
                 }
             }