chore: skip timing-sensistive AgentMetadata test in the standard suite (#7237)

* chore: skip timing-sensistive AgentMetadata test in the standard suite

* Add test-timing target

* fix windows?

* Works on my Windows desktop?

* Use tag system

* fixup! Use tag system

testutil/enable_timing.go

@@ -0,0 +1,8 @@
+//go:build timing
+
+package testutil
+
+var _ = func() any {
+	timing = true
+	return nil
+}()

testutil/timing.go

@@ -0,0 +1,20 @@
+package testutil
+
+import (
+	"testing"
+)
+
+// We can't run timing-sensitive tests in CI because of the
+// great variance in runner performance. Instead of not testing timing at all,
+// we relegate it to humans manually running certain tests with the "-timing"
+// flag from time to time.
+//
+// Eventually, we should run all timing tests in a self-hosted runner.
+
+var timing bool
+
+func SkipIfNotTiming(t *testing.T) {
+	if !timing {
+		t.Skip("skipping timing-sensitive test")
+	}
+}