[SPARK-47724][PYTHON][TESTS] Add an environment variable for testing …

…remote pure Python library

### What changes were proposed in this pull request?

This PR proposes to add an environment variable called `SPARK_CONNECT_TESTING_REMOTE` to set `remote` URL.

### Why are the changes needed?

In order to test pure Python library with a remote server.

### Does this PR introduce _any_ user-facing change?

No, test-only.

### How was this patch tested?

Manually tested.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#45868 from HyukjinKwon/SPARK-47724.

Authored-by: Hyukjin Kwon <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

python/pyspark/testing/connectutils.py

@@ -174,7 +174,7 @@ def conf(cls):
 
     @classmethod
     def master(cls):
-        return "local[4]"
+        return os.environ.get("SPARK_CONNECT_TESTING_REMOTE", "local[4]")
 
     @classmethod
     def setUpClass(cls):