vfs: bring open_tests.go generator back into line with the generated tests

In

54deb01f00 vfs: Make OpenFile and friends return EINVAL if O_RDONLY and O_TRUNC

The generated file open_test.go was edited directly without editing
the generator.

This commit brings the generator make_open_tests.go back into line
with that edit. It also makes it so `go generate` can be used to
regenerate the tests.

vfs/vfs.go

@@ -16,6 +16,9 @@
 // error conditions have ocurred.  It may also return general errors
 // it receives.  It tries to use os Error values (eg os.ErrExist)
 // where possible.
+
+//go:generate sh -c "go run make_open_tests.go | gofmt > open_test.go"
+
 package vfs
 
 import (