Update test-context dependency to 0.3

[arpad-m]

Updates the test-context dev-dependency of the remote_storage crate to 0.3. This removes a lot of async_trait instances.

Related earlier work: #6305, #6464

[github-actions]

2748 tests run: 2624 passed, 0 failed, 124 skipped (full report)

---

Flaky tests (1)

Postgres 15

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 27.9% (6381 of 22833 functions)
• lines: 46.8% (44986 of 96049 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
eacfbc8 at 2024-04-04T11:09:15.621Z :recycle:}

[koivunej]

build failed for the h2 rustsec... I'll bump it unless you get it done first.