path: improve documentation for Dir

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6495059
diff --git a/src/pkg/path/path_test.go b/src/pkg/path/path_test.go
index 109005d..0f353be 100644
--- a/src/pkg/path/path_test.go
+++ b/src/pkg/path/path_test.go
@@ -200,6 +200,7 @@
 	{"x/", "x"},
 	{"abc", "."},
 	{"abc/def", "abc"},
+	{"abc////def", "abc"},
 	{"a/b/.x", "a/b"},
 	{"a/b/c.", "a/b"},
 	{"a/b/c.x", "a/b"},