tstest: make ResourceCheck panic on parallel tests

To find potential flakes earlier.

Updates #deflake-effort

Change-Id: I52add6111d660821c3a23d4b1dd032821344bc48
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/resource.go

@@ -13,8 +13,19 @@ import (
 	"github.com/google/go-cmp/cmp"
 )
 
+// ResourceCheck takes a snapshot of the current goroutines and registers a
+// cleanup on tb to verify that after the rest, all goroutines created by the
+// test go away. (well, at least that the count matches. Maybe in the future it
+// can look at specific routines).
+//
+// It panics if called from a parallel test.
 func ResourceCheck(tb testing.TB) {
 	tb.Helper()
+
+	// Set an environment variable (anything at all) just for the
+	// side effect of tb.Setenv panicking if we're in a parallel test.
+	tb.Setenv("TS_CHECKING_RESOURCES", "1")
+
 	startN, startStacks := goroutines()
 	tb.Cleanup(func() {
 		if tb.Failed() {