vfstests: move functional tests from mountlib and make them work with VFS

The tests are now run for the mount commands and for the plain VFS.

This makes the tests much easier to debug when running with a VFS than
through a mount.

vfs/vfstest/read_unix.go

@@ -0,0 +1,53 @@
+// +build linux darwin freebsd
+
+package vfstest
+
+import (
+	"syscall"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestReadFileDoubleClose tests double close on read
+func TestReadFileDoubleClose(t *testing.T) {
+	run.skipIfVFS(t)
+	run.skipIfNoFUSE(t)
+
+	run.createFile(t, "testdoubleclose", "hello")
+
+	in, err := run.os.Open(run.path("testdoubleclose"))
+	assert.NoError(t, err)
+	fd := in.Fd()
+
+	fd1, err := syscall.Dup(int(fd))
+	assert.NoError(t, err)
+
+	fd2, err := syscall.Dup(int(fd))
+	assert.NoError(t, err)
+
+	// close one of the dups - should produce no error
+	err = syscall.Close(fd1)
+	assert.NoError(t, err)
+
+	// read from the file
+	buf := make([]byte, 1)
+	_, err = in.Read(buf)
+	assert.NoError(t, err)
+
+	// close it
+	err = in.Close()
+	assert.NoError(t, err)
+
+	// read from the other dup - should produce no error as this
+	// file is now buffered
+	n, err := syscall.Read(fd2, buf)
+	assert.NoError(t, err)
+	assert.Equal(t, 1, n)
+
+	// close the dup - should not produce an error
+	err = syscall.Close(fd2)
+	assert.NoError(t, err, "input/output error")
+
+	run.rm(t, "testdoubleclose")
+}