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

Dockerfile

@@ -29,7 +29,10 @@ RUN go mod download
 
 COPY . .
 
-RUN go test -count=1 -timeout 90s -race -cover ./... || \
+# Go's build cache is kept on a tmpfs, out of the image: nothing uses it
+# after this step, and writing it into the image takes seconds.
+RUN --mount=type=tmpfs,target=/root/.cache/go-build \
+    go test -count=1 -timeout 90s -race -cover ./... || \
     { echo "--- Rerunning with -v for details ---"; \
       go test -count=1 -timeout 90s -race -v ./...; exit 1; }
 

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
+	// Every timeout started by the time smallwebwaf read the request:
-	// them all, then use the connection.
+	// wait until just past shortTimeout after it was sent, then use the
-	time.Sleep(3 * shortTimeout / 2)
+	// connection.
+	time.Sleep(time.Until(sent.Add(shortTimeout + 100*time.Millisecond)))
 	send(t, conn, "still here\n")
 
 	echoed, err := reader.ReadString('\n')

internal/proxy/proxy_test.go

@@ -23,9 +23,9 @@ const (
 	// shortTimeout is what a test sets a timeout to, to see it run out.
 	// It starts before the test has set up its case, such as an upgrade
 	// or the app's buffers filling, so it is as long as the hold-up of the
-	// test process that wantTimedOut allows, waitLimit/2: a shorter one
+	// test process that wantTimedOut allows: a shorter one can run out
-	// can run out first on a busy host.
+	// first on a busy host.
-	shortTimeout = 5 * time.Second
+	shortTimeout = waitLimit / 2
 	// shortTimeoutSetting is shortTimeout as a setting's value.
 	shortTimeoutSetting = "5s"
 	// longTimeoutSetting is a timeout that does not run out in a test.