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/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.