nettest, *: add option to run HTTP tests with in-memory network

To avoid ephemeral port / TIME_WAIT exhaustion with high --count
values, and to eventually detect leaked connections in tests. (Later
the memory network will register a Cleanup on the TB to verify that
everything's been shut down)

Updates tailscale/corp#27636

Change-Id: Id06f1ae750d8719c5a75d871654574a8226d2733
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/testenv/testenv.go

@@ -58,3 +58,10 @@ func InParallelTest(t TB) (isParallel bool) {
 	t.Chdir(".") // panics in a t.Parallel test
 	return false
 }
+
+// AssertInTest panics if called outside of a test binary.
+func AssertInTest() {
+	if !InTest() {
+		panic("func called outside of test binary")
+	}
+}