extend tests for `strings` module

vlib/strings/strings_test.v

@@ -1,6 +1,7 @@
 import strings
 
 fn test_repeat() {
-	assert strings.repeat(`x`, 10) == 'xxxxxxxxxx' 
-	assert strings.repeat(`a`, 1) == 'a' 
-} 
+	assert strings.repeat(`x`, 10) == 'xxxxxxxxxx'
+	assert strings.repeat(`a`, 1) == 'a'
+	assert strings.repeat(`a`, 0) == ''
+}