test: fix TestServer_X11_EvictionLRU hang on fish shell (#23838)

`TestServer_X11_EvictionLRU` hangs forever when the developer's login
shell is `fish`. This is the only test in the repo that breaks on fish,
and it meant I couldn't run `make test` or similar without it blocking
indefinitely.

The test uses `sess.Shell()` to start interactive shell sessions, which
causes the SSH server to run the user's login shell directly (`fish
-l`). Fish buffers all piped stdin to EOF before executing any of it, so
the test's `echo ready-0\n` write never gets processed — fish sits
waiting for the pipe to close, and the test sits waiting for the echo
response.

The fix is a one-line change: `sess.Shell()` → `sess.Start("sh")`. The
test is exercising X11 LRU eviction, not shell behavior, so using `sh`
explicitly is both correct and shell-agnostic. The DISPLAY environment
variable is set identically either way since the x11-req handler runs
before `sessionStart`.

agent/agentssh/x11_test.go

@@ -211,7 +211,7 @@ func TestServer_X11_EvictionLRU(t *testing.T) {
 		require.NoError(t, err)
 		stderr, err := sess.StderrPipe()
 		require.NoError(t, err)
-		require.NoError(t, sess.Shell())
+		require.NoError(t, sess.Start("sh"))
 
 		// The SSH server lazily starts the session. We need to write a command
 		// and read back to ensure the X11 forwarding is started.