pageserver: fix tenant::storage_layer::layer tests

[erikgrinaker]

Problem

cargo test fails non-deterministically on these tests, because they share global state via LAYER_IMPL_METRICS and then assert on this global state.

Tests really should not share global state, but as a simple hack for now, just reset the global state and serialize the tests.

Summary of changes

Reset LAYER_IMPL_METRICS for each test, and run tests serially.

[github-actions]

7095 tests run: 6796 passed, 0 failed, 299 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_create_churn_during_restart: debug-x86-64

Postgres 15

• test_check_visibility_map: release-arm64

Code coverage* (full report)

• functions: 31.3% (8413 of 26889 functions)
• lines: 48.0% (66729 of 139096 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7475bf4 at 2024-12-23T12:55:25.314Z :recycle:}