Adopt databricks-labs-pytester PyPI package (#2663)

This PR moves fixture usage from `mixins.fixtures` into its own
top-level library.

Author: nfx

pyproject.toml

@@ -77,7 +77,8 @@ dependencies = [
     "pylint~=3.2.2",
     "pylint-pytest==2.0.0a0",
     "databricks-labs-pylint~=0.4.0",
-    "pytest~=8.1.0",
+    "databricks-labs-pytester>=0.2.1",
+    "pytest~=8.3.3",
     "pytest-cov~=4.1.0",
     "pytest-mock~=3.14.0",
     "pytest-timeout~=2.3.1",