x/auth: turn sign --validate-sigantures into a standalone command (#6108)

--validate-signatures should not be a flag of the sign command
as the operation performed (transaction signatures verification)
is logically distinct.

cli_test is and has always been an horrible name for package
directory as it's very much Go anti-idiomatic - _test is the
suffix used by test packages, not directories. Plus, CLI test
cases can and should live alongside other testcases that don't
require binaries to be built beforehand. Thus:

x/module/client/cli_test/*.go -> x/module/client/cli/

Test files that require sim{cli,d} shall be tagged with // +build cli_test

With regard to cli test auxiliary functions, they should live in:

x/module/client/testutil/

Co-authored-by: Alexander Bezobchuk <alexanderbez@users.noreply.github.com>

simapp/cmd/simcli/main.go

@@ -124,6 +124,7 @@ func txCmd(cdc *amino.Codec) *cobra.Command {
 		flags.LineBreak,
 		authcmd.GetSignCommand(cdc),
 		authcmd.GetMultiSignCommand(cdc),
+		authcmd.GetValidateSignaturesCommand(cdc),
 		flags.LineBreak,
 		authcmd.GetBroadcastCommand(cdc),
 		authcmd.GetEncodeCommand(cdc),