tstest/integration: fix filch test flake

Filch doesn't like having multiple processes competing
for the same log files (#937).

Parallel integration tests were all using the same log files.

Add a TS_LOGS_DIR env var that the integration test can use
to use separate log files per test.

Fixes #2269

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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