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/write_unix.go

@@ -0,0 +1,69 @@
+// +build linux darwin freebsd
+
+package vfstest
+
+import (
+	"runtime"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/sys/unix"
+
+	"github.com/rclone/rclone/vfs"
+)
+
+// TestWriteFileDoubleClose tests double close on write
+func TestWriteFileDoubleClose(t *testing.T) {
+	run.skipIfVFS(t)
+	run.skipIfNoFUSE(t)
+	if runtime.GOOS == "darwin" {
+		t.Skip("Skipping test on OSX")
+	}
+
+	out, err := osCreate(run.path("testdoubleclose"))
+	assert.NoError(t, err)
+	fd := out.Fd()
+
+	fd1, err := unix.Dup(int(fd))
+	assert.NoError(t, err)
+
+	fd2, err := unix.Dup(int(fd))
+	assert.NoError(t, err)
+
+	// close one of the dups - should produce no error
+	err = unix.Close(fd1)
+	assert.NoError(t, err)
+
+	// write to the file
+	buf := []byte("hello")
+	n, err := out.Write(buf)
+	assert.NoError(t, err)
+	assert.Equal(t, 5, n)
+
+	// close it
+	err = out.Close()
+	assert.NoError(t, err)
+
+	// write to the other dup
+	_, err = unix.Write(fd2, buf)
+	if run.vfs.Opt.CacheMode < vfs.CacheModeWrites {
+		// produces an error if cache mode < writes
+		assert.Error(t, err, "input/output error")
+	} else {
+		// otherwise does not produce an error
+		assert.NoError(t, err)
+	}
+
+	// close the dup - should not produce an error
+	err = unix.Close(fd2)
+	assert.NoError(t, err)
+
+	run.waitForWriters()
+	run.rm(t, "testdoubleclose")
+}
+
+// writeTestDup performs the platform-specific implementation of the dup() unix
+func writeTestDup(oldfd uintptr) (uintptr, error) {
+	newfd, err := unix.Dup(int(oldfd))
+	return uintptr(newfd), err
+}