[SPARK-26148][PYTHON][TESTS] Increases default parallelism in PySpark tests to speed up

## What changes were proposed in this pull request?

This PR proposes to increase parallelism in PySpark tests to speed up from 4 to 8.

It decreases the elapsed time from

https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/99163/consoleFull
Tests passed in 1770 seconds

to

https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/99186/testReport/
Tests passed in 1027 seconds

## How was this patch tested?

Jenkins tests

Closes apache#23111 from HyukjinKwon/parallelism.

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

Author: HyukjinKwon

dev/run-tests.py

@@ -460,7 +460,7 @@ def parse_opts():
         prog="run-tests"
     )
     parser.add_option(
-        "-p", "--parallelism", type="int", default=4,
+        "-p", "--parallelism", type="int", default=8,
         help="The number of suites to test in parallel (default %default)"
     )