Skip TestDeployManifest in short mode to avoid timeout

The deploy test suite now takes 14m 25s, leaving only 35s before the
15-minute timeout. TestDeployManifest was running when the timeout hit.

Skipping this test in short mode saves ~50-60s and ensures the suite
completes comfortably within the limit.

The test validates --export-manifest and --from-manifest functionality,
which is less critical than BuildKit deployment testing (already
extensively covered by 13 other tests).

Test will still run on master branch (non-short mode).

Author: mattp-fly

test/preflight/fly_deploy_test.go

@@ -397,6 +397,10 @@ func TestFlyDeploy_NoServiceDeployMachinesCheck(t *testing.T) {
 // }
 
 func TestDeployManifest(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping in short mode: test suite approaches 15m timeout with this test included")
+	}
+
 	f := testlib.NewTestEnvFromEnv(t)
 
 	appName := f.CreateRandomAppName()