test: add SQL file tests for regr_* linear-regression aggregates

[andygrove]

Which issue does this PR close?

N/A. This adds test coverage only.

Rationale for this change

The SQL standard linear-regression aggregates (regr_count, regr_avgx, regr_avgy, regr_sxx, regr_syy, regr_sxy, regr_slope, regr_intercept, regr_r2) had no Comet test coverage. Comet already accelerates several of them natively, because Spark lowers them to Count / Average aggregates that Comet supports, but this was unverified. These tests establish which of the family run natively in Comet versus fall back to Spark, and guard the accelerated ones against future regressions.

What changes are included in this PR?

Adds one SQL file test, spark/src/test/resources/sql-tests/expressions/aggregate/regr.sql, covering all nine regr_* functions over global aggregates, GROUP BY, all-NULL input, and single-pair input (only rows where both y and x are non-null contribute).

• regr_count, regr_avgx, regr_avgy run natively. They use the default query mode, which asserts the query executes on Comet without falling back to Spark and matches Spark exactly.
• regr_sxx, regr_syy, regr_sxy, regr_slope, regr_intercept, regr_r2 currently fall back to Spark, so they use query spark_answer_only to validate result correctness against Spark.

How are these changes tested?

This PR is test-only. The new file runs under CometSqlFileTestSuite, which executes each query through both Spark and Comet and compares results; the default query blocks additionally assert native (non-fallback) Comet execution. Verified locally:

./mvnw test -Dsuites="org.apache.comet.CometSqlFileTestSuite regr" -Dtest=none

Result: the single discovered test file passes.

[mbutrovich]

Tests are good! Thanks @andygrove!