cmd/ethkey: fix formatting, review nits (#15807)

This commit:

- Adds a --msgfile option to read the message to sign from a file
  instead of command line argument.
- Adds a unit test for signing subcommands.
- Removes some weird whitespace in the code.

cmd/ethkey/main.go

@@ -28,30 +28,11 @@ const (
 	defaultKeyfileName = "keyfile.json"
 )
 
-var (
-	gitCommit = "" // Git SHA1 commit hash of the release (set via linker flags)
+// Git SHA1 commit hash of the release (set via linker flags)
+var gitCommit = ""
 
-	app *cli.App // the main app instance
-)
+var app *cli.App
 
-var ( // Commonly used command line flags.
-	passphraseFlag = cli.StringFlag{
-		Name:  "passwordfile",
-		Usage: "the file that contains the passphrase for the keyfile",
-	}
-
-	jsonFlag = cli.BoolFlag{
-		Name:  "json",
-		Usage: "output JSON instead of human-readable format",
-	}
-
-	messageFlag = cli.StringFlag{
-		Name:  "message",
-		Usage: "the file that contains the message to sign/verify",
-	}
-)
-
-// Configure the app instance.
 func init() {
 	app = utils.NewApp(gitCommit, "an Ethereum key manager")
 	app.Commands = []cli.Command{
@@ -62,6 +43,22 @@ func init() {
 	}
 }
 
+// Commonly used command line flags.
+var (
+	passphraseFlag = cli.StringFlag{
+		Name:  "passwordfile",
+		Usage: "the file that contains the passphrase for the keyfile",
+	}
+	jsonFlag = cli.BoolFlag{
+		Name:  "json",
+		Usage: "output JSON instead of human-readable format",
+	}
+	messageFlag = cli.StringFlag{
+		Name:  "message",
+		Usage: "the file that contains the message to sign/verify",
+	}
+)
+
 func main() {
 	if err := app.Run(os.Args); err != nil {
 		fmt.Fprintln(os.Stderr, err)