fix(Makefile): align test-race with CI configuration (#22727)

Follow-up to #22705 (pre-commit/pre-push hooks).

Unifies `test` and `test-race` into the same structure and lets CI call
`make test-race` instead of reproducing the gotestsum command.

**Parallelism**: Extracted from `GOTEST_FLAGS` into
`TEST_PARALLEL_PACKAGES`
/ `TEST_PARALLEL_TESTS` (default 8x8). `test-race` overrides to 4x4 via
target-specific Make variables. `TEST_NUM_PARALLEL_PACKAGES` and
`TEST_NUM_PARALLEL_TESTS` env vars continue to work for both targets.

**GOTEST_FLAGS**: Changed from simply-expanded (`:=`) to
recursively-expanded
(`=`) so target-specific overrides take effect at recipe time.

**CI**: `.github/actions/test-go-pg/action.yaml` now calls `make
test-race`
/ `make test` instead of hand-rolling the gotestsum command, eliminating
drift between local and CI configurations.

Refs #22705

.github/actions/test-go-pg/action.yaml

@@ -70,11 +70,7 @@ runs:
         set -euo pipefail
 
         if [[ ${RACE_DETECTION} == true ]]; then
-          gotestsum --junitfile="gotests.xml" --packages="${TEST_PACKAGES}" -- \
-            -tags=testsmallbatch \
-            -race \
-            -parallel "${TEST_NUM_PARALLEL_TESTS}" \
-            -p "${TEST_NUM_PARALLEL_PACKAGES}"
+          make test-race
         else
           make test
         fi