[Spark] Set spark.testing flag in tests

[ctring]

Which Delta project/connector is this regarding?

• Spark
• Standalone
• Flink
• Kernel
• Other (fill in here)

Description

Currently, the Utils.isTesting check in tests is not reliable because the spark.testing system property is not set before the suites start, so the tests are relying on this line in SparkFunSuite to set the test indicator. This is causing issues in CI because it means whether Utils.isTesting returns correct result depends on whether it is called after SparkFunSuite::beforeAll is called.

For example, some deletion vector-related suites are relying on the TEST_DV_NAME_PREFIX config to be correctly initialized in tests. However, if a previous test in the same JVM group initialized this config before spark.testing is correctly set by SparkFunSuite, the deletion vector suites would see the incorrect value of TEST_DV_NAME_PREFIX and fail (This is currently not an issue in master by luck but if a PR added new tests that changed the JVM grouping in CI job (#4039, #4040), this would incorrectly fail the PR's spark test job).

This PR fixes this by setting spark.testing to true in build file. Note that this flag is already set in the Spark repo but not here yet.

How was this patch tested?

Check that the TEST_DV_NAME_PREFIX is correctly set for tests that run Utils.testing before SparkFunSuite::beforeAll (e.g., org.apache.spark.sql.delta.DeltaWithNewTransactionWithCoordinatedCommitsBatch100Suite)

Does this PR introduce any user-facing changes?

No

[felipepessoto]

@ctring could you help reviewing this related PR? #4074