Spend less of make test writing the image and waiting (closes #56)

The test phase spends most of its time compiling with the race
detector from an empty build cache; then come writing the test image
and the internal/proxy tests.

- Go's build cache is on a tmpfs in the test phase, so its 137 MB are
  no longer written into the test image.
- TestUpgradedConnectionOutlastsTheTimeouts waits until just past
  shortTimeout after the request was sent, rather than 7.5 s after the
  upgrade, so it ends with the other timing tests.
- shortTimeout is written as waitLimit / 2, as its comment says it is.

Model: opus-5-5

internal/proxy/passthrough_test.go

@@ -233,6 +233,7 @@ func TestUpgradedConnectionOutlastsTheTimeouts(t *testing.T) {
 	send(t, conn, "GET /socket HTTP/1.1\r\nHost: app\r\n"+
 		"Connection: Upgrade\r\nUpgrade: websocket\r\n\r\n")
 
+	sent := time.Now()
 	reader := bufio.NewReader(conn)
 
 	res, err := http.ReadResponse(reader, nil)
@@ -246,9 +247,10 @@ func TestUpgradedConnectionOutlastsTheTimeouts(t *testing.T) {
 		t.Fatalf("status %d, want %d", res.StatusCode, http.StatusSwitchingProtocols)
 	}
 
-	// Every timeout started before the upgrade was answered: wait past
-	// them all, then use the connection.
-	time.Sleep(3 * shortTimeout / 2)
+	// Every timeout started by the time smallwebwaf read the request:
+	// wait until just past shortTimeout after it was sent, then use the
+	// connection.
+	time.Sleep(time.Until(sent.Add(shortTimeout + 100*time.Millisecond)))
 	send(t, conn, "still here\n")
 
 	echoed, err := reader.ReadString('\n')