test_runner: skip more tests using decorator instead of pytest.skip (#9704)

## Problem

Running `pytest.skip(...)` in a test body instead of marking the test
with `@pytest.mark.skipif(...)` makes all fixtures to be initialised,
which is not necessary if the test is going to be skipped anyway.

Also, some tests are unnecessarily skipped (e.g. `test_layer_bloating`
on Postgres 17, or `test_idle_reconnections` at all) or run (e.g.
`test_parse_project_git_version_output_positive` more than on once
configuration) according to comments.

## Summary of changes
- Move `skip_on_postgres` / `xfail_on_postgres` /
`run_only_on_default_postgres` decorators to `fixture.utils`
- Add new `skip_in_debug_build` and `skip_on_ci` decorators
- Replace `pytest.skip(...)` calls with decorators where possible

test_runner/regress/test_storage_controller.py

@@ -36,11 +36,12 @@ from fixtures.pageserver.utils import (
     remote_storage_delete_key,
     timeline_delete_wait_completed,
 )
-from fixtures.pg_version import PgVersion, run_only_on_default_postgres
+from fixtures.pg_version import PgVersion
 from fixtures.port_distributor import PortDistributor
 from fixtures.remote_storage import RemoteStorageKind, s3_storage
 from fixtures.storage_controller_proxy import StorageControllerProxy
 from fixtures.utils import (
+    run_only_on_default_postgres,
     run_pg_bench_small,
     subprocess_capture,
     wait_until,