Increase cpp-memcheck test timeout (rapidsai#21901)

Increases the timeout for the nightly compute-sanitizer memcheck runs from 3 hours to 3.5 hours.
The nightly runs usually complete in about 2.5 hours but some slower runners will hit the timeout and I've estimated an additional 10 minutes would suffice. Changing to 3.5 hours also allows a buffer for future tests.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: rapidsai#21901

Author: davidwendt

ci/test_cpp_memcheck.sh

@@ -11,7 +11,7 @@ source ./ci/test_cpp_common.sh
 
 rapids-logger "Memcheck gtests with rmm_mode=cuda"
 
-timeout 3h ./ci/run_cudf_memcheck_ctests.sh && EXITCODE=$? || EXITCODE=$?;
+timeout 3.5h ./ci/run_cudf_memcheck_ctests.sh && EXITCODE=$? || EXITCODE=$?;
 
 rapids-logger "Test script exiting with value: $EXITCODE"
 # shellcheck disable=SC2086