GH-43868: [CI][Python] Skip test that requires PARQUET_TEST_DATA env …

…on emscripten (#43906)

### Rationale for this change

The following PR:
- #41580

Made mandatory for a test the requirement to have `PARQUET_TEST_DATA` env defined.

This is currently not available from `python_test_emscripten.sh` as we require to mount the filesystem for both Node and ChromeDriver.

### What changes are included in this PR?

Skip the test that requires `PARQUET_TEST_DATA` for emscripten.

### Are these changes tested?

Via archery

### Are there any user-facing changes?

No
* GitHub Issue: #43905
* GitHub Issue: #43868

Lead-authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

python/pyarrow/tests/parquet/conftest.py

@@ -17,6 +17,7 @@
 
 import os
 import pathlib
+import sys
 
 import pytest
 
@@ -30,6 +31,8 @@ def datadir(base_datadir):
 
 @pytest.fixture(scope='module')
 def parquet_test_datadir():
+    if sys.platform == 'emscripten':
+        pytest.skip("needs PARQUET_TEST_DATA files access")
     result = os.environ.get('PARQUET_TEST_DATA')
     if not result:
         raise RuntimeError('Please point the PARQUET_TEST_DATA environment '