[MINOR] Enable PySpark SQL readerwriter and window tests

PySpark SQL's `readerwriter` and `window` doctests weren't being run by our test runner script; this patch re-enables them.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #6542 from JoshRosen/enable-more-pyspark-sql-tests and squashes the following commits:

9f46ce4 [Josh Rosen] Enable PySpark SQL readerwriter and window tests.

python/run-tests

@@ -76,6 +76,8 @@ function run_sql_tests() {
     run_test "pyspark.sql.dataframe"
     run_test "pyspark.sql.group"
     run_test "pyspark.sql.functions"
+    run_test "pyspark.sql.readwriter"
+    run_test "pyspark.sql.window"
     run_test "pyspark.sql.tests"
 }