[SPARK-17524][TESTS] Use specified spark.buffer.pageSize

## What changes were proposed in this pull request?

This PR has the appendRowUntilExceedingPageSize test in RowBasedKeyValueBatchSuite use whatever spark.buffer.pageSize value a user has specified to prevent a test failure for anyone testing Apache Spark on a box with a reduced page size. The test is currently hardcoded to use the default page size which is 64 MB so this minor PR is a test improvement

## How was this patch tested?
Existing unit tests with 1 MB page size and with 64 MB (the default) page size

Author: Adam Roberts <aroberts@uk.ibm.com>

Closes #15079 from a-roberts/patch-5.