cmd/tailscale/cli: exit node filter should display all exit node options (#12699)

This change expands the `exit-node list -filter` command to display all
location based exit nodes for the filtered country. This allows users
to switch to alternative servers when our recommended exit node is not
working as intended.

This change also makes the country filter matching case insensitive,
e.g. both USA and usa will work.

Updates #12698

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

cmd/tailscale/cli/exitnode_test.go

@@ -219,7 +219,7 @@ func TestFilterFormatAndSortExitNodes(t *testing.T) {
 						{
 							Name: "Rainier",
 							Peers: []*ipnstate.PeerStatus{
-								ps[2],
+								ps[2], ps[3],
 							},
 						},
 					},