e2e: start a new file for cli plugins arguments tests

`TestRunGoodArgument` fits here.

Signed-off-by: Ian Campbell <ijc@docker.com>

e2e/cli-plugins/flags_test.go

@@ -0,0 +1,19 @@
+package cliplugins
+
+import (
+	"testing"
+
+	"gotest.tools/icmd"
+)
+
+// TestRunGoodArgument ensures correct behaviour when running a valid plugin with an `--argument`.
+func TestRunGoodArgument(t *testing.T) {
+	run, _, cleanup := prepare(t)
+	defer cleanup()
+
+	res := icmd.RunCmd(run("helloworld", "--who", "Cleveland"))
+	res.Assert(t, icmd.Expected{
+		ExitCode: 0,
+		Out:      "Hello Cleveland!",
+	})
+}

e2e/cli-plugins/run_test.go

@@ -144,18 +144,6 @@ func TestRunGoodSubcommand(t *testing.T) {
 	})
 }
 
-// TestRunGoodArgument ensures correct behaviour when running a valid plugin with an `--argument`.
-func TestRunGoodArgument(t *testing.T) {
-	run, _, cleanup := prepare(t)
-	defer cleanup()
-
-	res := icmd.RunCmd(run("helloworld", "--who", "Cleveland"))
-	res.Assert(t, icmd.Expected{
-		ExitCode: 0,
-		Out:      "Hello Cleveland!",
-	})
-}
-
 // TestHelpGoodSubcommand ensures correct behaviour when invoking help on a
 // valid plugin subcommand. A global argument is included to ensure it does not
 // interfere.