mounttest: fix unreliable tests on Windows CI

The failure is this which is not reproducable locally, only on the CI
servers.

    --- FAIL: TestMount/CacheMode=minimal/TestWriteFileOverwrite (1.01s)
        fs.go:351:
            Error Trace:    fs.go:351
                            write.go:65
            Error:          Received unexpected error:
                            open E:testwrite: The request could not be performed because of an I/O device error.
            Test:           TestMount/CacheMode=minimal/TestWriteFileOverwrite

The corresponding ERROR from the log is this:

    ERROR : IO error: truncate C:\Users\runneradmin\AppData\Local\rclone\vfs\local\C\Users\RUNNER~1\AppData\Local\Temp\rclone298719627\testwrite: Access is denied.

Instead of using ioutil.WriteFile this fix uses an equivalent based on
rclone's lib/file which doesn't set the exclusive flag on
Windows. This allows files to be deleted that are open.  It also
deletes existing files if an error is received and retries.

cmd/mountlib/mounttest/fs.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"flag"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"log"
 	"os"
@@ -22,6 +23,7 @@ import (
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/walk"
 	"github.com/rclone/rclone/fstest"
+	"github.com/rclone/rclone/lib/file"
 	"github.com/rclone/rclone/vfs"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -345,9 +347,36 @@ func (r *Run) waitForWriters() {
 	run.vfs.WaitForWriters(10 * time.Second)
 }
 
+// writeFile writes data to a file named by filename.
+// If the file does not exist, WriteFile creates it with permissions perm;
+// otherwise writeFile truncates it before writing.
+// If there is an error writing then writeFile
+// deletes it an existing file and tries again.
+func writeFile(filename string, data []byte, perm os.FileMode) error {
+	f, err := file.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		err = os.Remove(filename)
+		if err != nil {
+			return err
+		}
+		f, err = file.OpenFile(filename, os.O_WRONLY|os.O_CREATE, perm)
+		if err != nil {
+			return err
+		}
+	}
+	n, err := f.Write(data)
+	if err == nil && n < len(data) {
+		err = io.ErrShortWrite
+	}
+	if err1 := f.Close(); err == nil {
+		err = err1
+	}
+	return err
+}
+
 func (r *Run) createFile(t *testing.T, filepath string, contents string) {
 	filepath = r.path(filepath)
-	err := ioutil.WriteFile(filepath, []byte(contents), 0600)
+	err := writeFile(filepath, []byte(contents), 0600)
 	require.NoError(t, err)
 	r.waitForWriters()
 }