test: don't use ptytest for client side of SSH session tests (#27681)

In our initial batches of test refactors, I left the SSH session tests using `ptytest` because I (erroneously) thought that we still needed a client side PTY when the SSH server creates a PTY. This is incorrect and plain in-process IO is fine on the client side.  
  
closes https://github.com/coder/internal/issues/1400   
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testutil/writer.go

@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"golang.org/x/crypto/ssh"
 	"gvisor.dev/gvisor/pkg/context"
 
 	"cdr.dev/slog/v3"
@@ -32,6 +33,20 @@ func NewWriterAttachedToInvocation(t *testing.T, logger slog.Logger, invocation
 	}
 }
 
+func NewWriterAttachedToSSHSession(t *testing.T, l slog.Logger, session *ssh.Session) *Writer {
+	r, w := io.Pipe()
+	session.Stdin = r
+	// Close the pipe at the end of the test to ensure any goroutine in the Invocation that reads from stdin won't leak.
+	t.Cleanup(func() {
+		_ = w.Close()
+	})
+	return &Writer{
+		t: t,
+		w: w,
+		l: l,
+	}
+}
+
 func (w *Writer) Write(r rune) {
 	w.t.Helper()
 	_, err := w.w.Write([]byte{byte(r)})