lib/random: unify random string generation into random.String

This was factored from fstest as we were including the testing
enviroment into the main binary because of it.

This was causing opening the browser to fail because of 8243ff8bc8.

fstest/fstest.go

@@ -27,6 +27,7 @@ import (
 	"github.com/rclone/rclone/fs/config"
 	"github.com/rclone/rclone/fs/hash"
 	"github.com/rclone/rclone/fs/walk"
+	"github.com/rclone/rclone/lib/random"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/text/unicode/norm"
@@ -357,24 +358,6 @@ func Time(timeString string) time.Time {
 	return t
 }
 
-// RandomString create a random string for test purposes
-func RandomString(n int) string {
-	const (
-		vowel     = "aeiou"
-		consonant = "bcdfghjklmnpqrstvwxyz"
-		digit     = "0123456789"
-	)
-	pattern := []string{consonant, vowel, consonant, vowel, consonant, vowel, consonant, digit}
-	out := make([]byte, n)
-	p := 0
-	for i := range out {
-		source := pattern[p]
-		p = (p + 1) % len(pattern)
-		out[i] = source[rand.Intn(len(source))]
-	}
-	return string(out)
-}
-
 // LocalRemote creates a temporary directory name for local remotes
 func LocalRemote() (path string, err error) {
 	path, err = ioutil.TempDir("", "rclone")
@@ -403,7 +386,7 @@ func RandomRemoteName(remoteName string) (string, string, error) {
 		if !strings.HasSuffix(remoteName, ":") {
 			remoteName += "/"
 		}
-		leafName = "rclone-test-" + RandomString(24)
+		leafName = "rclone-test-" + random.String(24)
 		if !MatchTestRemote.MatchString(leafName) {
 			log.Fatalf("%q didn't match the test remote name regexp", leafName)
 		}
@@ -432,7 +415,7 @@ func RandomRemote(remoteName string, subdir bool) (fs.Fs, string, func(), error)
 		if err != nil {
 			return nil, "", nil, err
 		}
-		remoteName += "/rclone-test-subdir-" + RandomString(8)
+		remoteName += "/rclone-test-subdir-" + random.String(8)
 	}
 
 	remote, err := fs.NewFs(remoteName)