cmd/evm: add tests for evm t8n (#23507)

internal/cmdtest/test_cmd.go

@@ -118,6 +118,13 @@ func (tt *TestCmd) Expect(tplsource string) {
 	tt.Logf("Matched stdout text:\n%s", want)
 }
 
+// Output reads all output from stdout, and returns the data.
+func (tt *TestCmd) Output() []byte {
+	var buf []byte
+	tt.withKillTimeout(func() { buf, _ = io.ReadAll(tt.stdout) })
+	return buf
+}
+
 func (tt *TestCmd) matchExactOutput(want []byte) error {
 	buf := make([]byte, len(want))
 	n := 0