ci: Import numpy before pyarrow in tests to resolve import warning

[chalmerlowe]

A PytestDeprecationWarning was occurring in several test files because pyarrow, when imported by pytest.importorskip, would fail to import numpy.core.multiarray.

This change addresses the warning by explicitly importing numpy before pytest.importorskip("pyarrow", ...) in the affected test files. This ensures that numpy is fully initialized before pyarrow attempts to use it, resolving the underlying import error.

tests/unit/test_table.py::TestRowIterator::test_to_arrow_w_empty_table
  /repo/python-bigquery/tests/unit/test_table.py:3270: PytestDeprecationWarning: 
  Module 'pyarrow' was found, but when imported by pytest it raised:

      **ImportError('numpy.core.multiarray failed to import')**

  In pytest 9.1 this warning will become an error by default.
  You can fix the underlying problem, or alternatively overwrite this behavior and silence this warning by passing exc_type=ImportError explicitly.
  See https://docs.pytest.org/en/stable/deprecations.html#pytest-importorskip-default-behavior-regarding-importerror
    pyarrow = pytest.importorskip(