Run a short replay outside the flake check

Run a short replay outside the flake check

The replay of the synthetic log runs 5,000 messages with -short and
100,000 messages without it. bin/check and the test run of nix build
pass -short, so bin/check takes about 20 seconds instead of more than
two minutes under the race detector. A new flake check runs only
TestReplaySyntheticLog without -short, so nix flake check still runs the
full replay and does not run the other tests a second time.

The test uses testing.Short, because -short is the standard Go flag to
make long tests shorter, and go test and buildGoModule pass it without
new code. An environment variable would need its own name and parsing
for the same result. The short replay gives the chat view 26 batches,
and the test still requires at least one batch.

Issue: e4109b7