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_replica_start.py

@@ -30,7 +30,7 @@ import pytest
 from fixtures.log_helper import log
 from fixtures.neon_fixtures import NeonEnv, wait_for_last_flush_lsn, wait_replica_caughtup
 from fixtures.pg_version import PgVersion
-from fixtures.utils import query_scalar, wait_until
+from fixtures.utils import query_scalar, skip_on_postgres, wait_until
 
 CREATE_SUBXACTS_FUNC = """
 create or replace function create_subxacts(n integer) returns void as $$
@@ -137,6 +137,12 @@ def test_replica_start_scan_clog_crashed_xids(neon_simple_env: NeonEnv):
     assert secondary_cur.fetchone() == (1,)
 
 
+@skip_on_postgres(
+    PgVersion.V14, reason="pg_log_standby_snapshot() function is available since Postgres 16"
+)
+@skip_on_postgres(
+    PgVersion.V15, reason="pg_log_standby_snapshot() function is available since Postgres 16"
+)
 def test_replica_start_at_running_xacts(neon_simple_env: NeonEnv, pg_version):
     """
     Test that starting a replica works right after the primary has
@@ -149,9 +155,6 @@ def test_replica_start_at_running_xacts(neon_simple_env: NeonEnv, pg_version):
     """
     env = neon_simple_env
 
-    if env.pg_version == PgVersion.V14 or env.pg_version == PgVersion.V15:
-        pytest.skip("pg_log_standby_snapshot() function is available only in PG16")
-
     primary = env.endpoints.create_start(branch_name="main", endpoint_id="primary")
     primary_conn = primary.connect()
     primary_cur = primary_conn.cursor()