Use TEST_OUTPUT envvar in pageserver

[bojanserafimov]

In python tests we can use TEST_OUTPUT envvar to overwrite the default dir. It's useful for perf tests where you want to write to a dir inside the mounted ssd (the neon repo itself might not be). I'm sure there are other workarounds but this seems generally useful anyway.

[github-actions]

2406 tests run: 2310 passed, 0 failed, 96 skipped (full report)

---

Flaky tests (3)

Postgres 16

• test_emergency_relocate_with_branches_slow_replay[local_fs]: debug

Postgres 15

• test_gc_cutoff: debug
• test_ondemand_download_timetravel[mock_s3]: debug

Code coverage (full report)

• functions: 54.5% (9179 of 16846 functions)
• lines: 82.1% (53503 of 65166 lines)

---

_{The comment gets automatically updated with the latest test results
bea0b13 at 2023-11-29T21:08:42.924Z :recycle:}

[problame]

Would prefer if we add an intermediary component like $TEST_OUTPUT_DIR/pageserver_rust_tests/test_{test_name}