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/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
-	// can run out first on a busy host.
-	shortTimeout = 5 * time.Second
+	// test process that wantTimedOut allows: a shorter one can run out
+	// first on a busy host.
+	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.