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 answer to the upgrade was read, by when every
  timeout has started, rather than 7.5 s, 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

@@ -233,7 +233,6 @@ 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)
@@ -241,16 +240,17 @@ func TestUpgradedConnectionOutlastsTheTimeouts(t *testing.T) {
 		t.Fatalf("read the answer to the upgrade: %v", err)
 	}
 
+	answered := time.Now()
 	_ = res.Body.Close()
 
 	if res.StatusCode != http.StatusSwitchingProtocols {
 		t.Fatalf("status %d, want %d", res.StatusCode, http.StatusSwitchingProtocols)
 	}
 
-	// Every timeout started by the time smallwebwaf read the request:
+	// Every timeout started before the upgrade was answered, the response
-	// wait until just past shortTimeout after it was sent, then use the
+	// timeouts last, at the end of the request: wait until just past
-	// connection.
+	// shortTimeout after the answer was read, then use the connection.
-	time.Sleep(time.Until(sent.Add(shortTimeout + 100*time.Millisecond)))
+	time.Sleep(time.Until(answered.Add(shortTimeout + 100*time.Millisecond)))
 	send(t, conn, "still here\n")
 
 	echoed, err := reader.ReadString('\n')