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

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; }