Make PRs changing pyproject.toml neutral for breeze tests (#37314)

The #37305 optimized CI jobs to limit tests run when pyproject.toml
is changed without impacting dependencies, optional_dependencies and
build-system. However it impacts results of other unit tests as they
were using `HEAD` as commit in selective checks,

Instead of using HEAD, we are using neutral commit (one only changing
INTHEWILD) for all such tests, so that we do not introduce side effects
coming from the incoming PR.

(cherry picked from commit 08e655a)

dev/breeze/tests/test_selective_checks.py

@@ -38,6 +38,9 @@
 ALL_DOCS_SELECTED_FOR_BUILD = ""
 ALL_PROVIDERS_AFFECTED = ""
 
+# commit that is neutral - allows to keep pyproject.toml-changing PRS neutral for unit tests
+NEUTRAL_COMMIT = "938f0c1f3cc4cbe867123ee8aa9f290f9f18100a"
+
 
 def escape_ansi_colors(line):
     return ANSI_COLORS_MATCHER.sub("", line)
@@ -587,7 +590,7 @@ def test_expected_output_pull_request_main(
 ):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         github_event=GithubEvents.PULL_REQUEST,
         pr_labels=(),
         default_branch="main",
@@ -687,7 +690,7 @@ def test_full_test_needed_when_scripts_changes(files: tuple[str, ...], expected_
     stderr = SelectiveChecks(
         files=files,
         github_event=GithubEvents.PULL_REQUEST,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         default_branch="main",
     )
     assert_outputs_are_printed(expected_outputs, str(stderr))
@@ -819,7 +822,7 @@ def test_expected_output_full_tests_needed(
 ):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         github_event=GithubEvents.PULL_REQUEST,
         pr_labels=pr_labels,
         default_branch=default_branch,
@@ -942,7 +945,7 @@ def test_expected_output_pull_request_v2_7(
 ):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         github_event=GithubEvents.PULL_REQUEST,
         pr_labels=(),
         default_branch="v2-7-stable",
@@ -1110,7 +1113,7 @@ def test_expected_output_pull_request_target(
 ):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         github_event=GithubEvents.PULL_REQUEST_TARGET,
         pr_labels=(),
         default_branch="main",
@@ -1199,7 +1202,7 @@ def test_expected_output_push(
 ):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         github_event=GithubEvents.PUSH,
         pr_labels=pr_labels,
         default_branch=default_branch,
@@ -1436,7 +1439,7 @@ def test_upgrade_to_newer_dependencies(
 def test_docs_filter(files: tuple[str, ...], expected_outputs: dict[str, str]):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         github_event=GithubEvents.PULL_REQUEST,
         pr_labels=(),
         default_branch="main",
@@ -1461,7 +1464,7 @@ def test_docs_filter(files: tuple[str, ...], expected_outputs: dict[str, str]):
 def test_helm_tests_trigger_ci_build(files: tuple[str, ...], expected_outputs: dict[str, str]):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         github_event=GithubEvents.PULL_REQUEST,
         pr_labels=(),
         default_branch="main",
@@ -1708,7 +1711,7 @@ def test_has_migrations(files: tuple[str, ...], has_migrations: bool):
     stderr = str(
         SelectiveChecks(
             files=files,
-            commit_ref="HEAD",
+            commit_ref=NEUTRAL_COMMIT,
             github_event=GithubEvents.PULL_REQUEST,
             default_branch="main",
         )
@@ -1742,7 +1745,7 @@ def test_has_migrations(files: tuple[str, ...], has_migrations: bool):
 def test_provider_compatibility_checks(labels: tuple[str, ...], expected_outputs: dict[str, str]):
     stderr = SelectiveChecks(
         files=(),
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         github_event=GithubEvents.PULL_REQUEST,
         pr_labels=labels,
         default_branch="main",
@@ -1830,7 +1833,7 @@ def test_mypy_matches(
 ):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         default_branch=default_branch,
         github_event=GithubEvents.PULL_REQUEST,
         pr_labels=pr_labels,
@@ -1878,7 +1881,7 @@ def test_pr_labels(
 ):
     stderr = SelectiveChecks(
         files=files,
-        commit_ref="HEAD",
+        commit_ref=NEUTRAL_COMMIT,
         default_branch="main",
         github_actor=github_actor,
         github_event=GithubEvents.PULL_REQUEST,