Move CometCollationSuite into the spark-4.1+ test shim to avoid per-version duplication

[0lai0]

Describe the bug / cleanup

In #4693, datetime expression collation tests were added to spark/src/test/spark-4.1/org/apache/spark/sql/CometCollationSuite.scala. Although these tests behave identically on Spark 4.1 and 4.2+, they currently reside in the spark-4.1 test shim, which applies only to the 4.1 profile. As noted in the review (#4693 (comment)), the recently introduced spark-4.1+ shim is the appropriate location for code shared across Spark 4.1 and all later 4.x releases. Keeping the suite in spark-4.1 would necessitate duplication once the spark-4.2 test profile is configured.

Additional context

Two things are needed, and neither is a trivial file copy:

1. The test build does not pick up the 4.1+ shim yet. add-test-source in spark/pom.xml only adds ${shims.majorVerSrc} and ${shims.minorVerSrc}; ${shims.minorPlusVerSrc} is wired into add-source (main sources) but not into the test sources.
2. CometCollationSuite already exists in the spark-4.1 shim, so the suite has to be moved rather than copied — having the same class in both spark-4.1 and spark-4.1+ would put two org.apache.spark.sql.CometCollationSuite classes on the 4.1 classpath and fail to compile.

The spark-4.0 suite should be left as-is: it carries 4.0-specific join tests and does not pick up the 4.1+ shim.

Proposed change

• Add <source>src/test/${shims.minorPlusVerSrc}</source> to theadd-test-source execution in spark/pom.xml.
• git mv the suite from spark/src/test/spark-4.1/... to spark/src/test/spark-4.1+/... (content unchanged).

Verified with:

• ./mvnw test -Pspark-4.1 -Dtest=none -Dsuites=org.apache.spark.sql.CometCollationSuite
• ./mvnw compile test-compile -Pspark-4.2 -DskipTests

[0lai0]

I'll work on this as the follow-up to the review comment on #4693.

cc @andygrove