[BUG] Mortgage unit tests fail with RAPIDS shuffle manager

[jlowe]

Nightly build failed in the unit tests across multiple Spark versions:

[2024-09-06T08:43:43.694Z] [INFO] --- scalatest-maven-plugin:2.0.2:test (test) @ rapids-4-spark-integration-tests_2.12 ---
[2024-09-06T08:43:44.255Z] Discovery starting.
[2024-09-06T08:43:44.255Z] Discovery completed in 286 milliseconds.
[2024-09-06T08:43:44.255Z] Run starting. Expected test count is: 12
[2024-09-06T08:43:44.512Z] MortgageSparkSuite:
[2024-09-06T08:43:44.772Z] RAPIDS SHUFFLE MANAGER ACTIVE
[2024-09-06T08:44:02.812Z] - extract mortgage data *** FAILED ***
[2024-09-06T08:44:02.812Z]   0 did not equal 10000 (MortgageSparkSuite.scala:65)

[jlowe]

GpuSubstring is being invoked during the failing test, so this may be the same root cause as #11437.