test: move map delete test to nil.go

No need for a separate test for this.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6941045
diff --git a/test/nil.go b/test/nil.go
index 6d90f50..44ca79e 100644
--- a/test/nil.go
+++ b/test/nil.go
@@ -147,6 +147,9 @@
 		panic(v)
 	}
 
+	// can delete (non-existent) entries
+	delete(m, 2)
+
 	// but cannot be written to
 	shouldPanic(func() {
 		m[2] = 3