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/fixtures/pg_version.py

@@ -1,10 +1,8 @@
 from __future__ import annotations
 
 import enum
-import os
 from typing import TYPE_CHECKING
 
-import pytest
 from typing_extensions import override
 
 if TYPE_CHECKING:
@@ -18,12 +16,15 @@ This fixture is used to determine which version of Postgres to use for tests.
 
 # Inherit PgVersion from str rather than int to make it easier to pass as a command-line argument
 # TODO: use enum.StrEnum for Python >= 3.11
-@enum.unique
 class PgVersion(str, enum.Enum):
     V14 = "14"
     V15 = "15"
     V16 = "16"
     V17 = "17"
+
+    # Default Postgres Version for tests that don't really depend on Postgres itself
+    DEFAULT = V16
+
     # Instead of making version an optional parameter in methods, we can use this fake entry
     # to explicitly rely on the default server version (could be different from pg_version fixture value)
     NOT_SET = "<-POSTRGRES VERSION IS NOT SET->"
@@ -59,27 +60,3 @@ class PgVersion(str, enum.Enum):
         # Make mypy happy
         # See https://github.com/python/mypy/issues/3974
         return None
-
-
-DEFAULT_VERSION: PgVersion = PgVersion.V16
-
-
-def skip_on_postgres(version: PgVersion, reason: str):
-    return pytest.mark.skipif(
-        PgVersion(os.environ.get("DEFAULT_PG_VERSION", DEFAULT_VERSION)) is version,
-        reason=reason,
-    )
-
-
-def xfail_on_postgres(version: PgVersion, reason: str):
-    return pytest.mark.xfail(
-        PgVersion(os.environ.get("DEFAULT_PG_VERSION", DEFAULT_VERSION)) is version,
-        reason=reason,
-    )
-
-
-def run_only_on_default_postgres(reason: str):
-    return pytest.mark.skipif(
-        PgVersion(os.environ.get("DEFAULT_PG_VERSION", DEFAULT_VERSION)) is not DEFAULT_VERSION,
-        reason=reason,
-    )