Make the load-sensitive tests wait for what they check (closes #507)

Three tests failed at random on a busy host. The browser test now waits for each page a click opens to load, and for Alpine.js to start on it, before reading it. The delivery tests' drain takes what is already queued instead of racing a 25 ms timer, since both dispatch paths queue before they return. The test phase keeps the tests' temporary directories on a tmpfs, because SQLite waiting for the disk made `internal/handlers` slow under load; the timeout and the parallel cap are unchanged.

Deviation: the delivery tests do not wait with a deadline; dispatch has finished when they read.
Unverified: `internal/handlers` at a host load of 260 to 290.

Model: opus-5-5

Dockerfile

@@ -147,9 +147,16 @@ RUN script/assets
 # shown in full, so there is nothing to rerun. The step fails after the rerun
 # whatever its result: the first run already showed the suite is broken.
 #
+# TMPDIR, where the tests keep their SQLite databases, is a tmpfs: SQLite
+# waits for the disk at every commit, and on a busy host that waiting was
+# about 40% of the slowest package's run time. GOTMPDIR keeps go's own
+# build files, the test binaries among them, on disk.
+#
 # bash with pipefail, so that the first run's status is go test's, not tee's.
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
-RUN go test -race -cover -p 4 -parallel 8 -timeout 90s ./... 2>&1 | tee /tmp/go-test.log && exit 0; \
+RUN --mount=type=tmpfs,target=/tmp/tests,size=512m \
+    export TMPDIR=/tmp/tests GOTMPDIR=/tmp; \
+    go test -race -cover -p 4 -parallel 8 -timeout 90s ./... 2>&1 | tee /tmp/go-test.log && exit 0; \
     tests="$(awk '/^--- FAIL: / { print $3 }' /tmp/go-test.log | paste -s -d '|' -)"; \
     packages="$(awk '/^FAIL\t/ { print $2 }' /tmp/go-test.log)"; \
     if [ -n "$tests" ]; then \