CI: Limit parallelism

[Fokko]

For the tests, we want to limit parallelism to avoid creating 1-row Parquet files.

[kevinjqliu]

LGTM

We previously added this config in dev/provision.py

iceberg-python/dev/provision.py

Lines 25 to 35 in 9945f83

	# The configuration is important, otherwise we get many small
	# parquet files with a single row. When a positional delete
	# hits the Parquet file with one row, the parquet file gets
	# dropped instead of having a merge-on-read delete file.
	spark = (
	SparkSession
	.builder
	.config("spark.sql.shuffle.partitions", "1")
	.config("spark.default.parallelism", "1")
	.getOrCreate()
	)

and looks like these are the only two places where we create SparkSession
https://grep.app/search?f.repo.pattern=iceberg-python&q=getOrCreate%28%29