fix(ci): restore testname format in sharded gotestsum runs (#36078)

run-shard-tests.sh called gotestsum directly without --format, so it
fell back to gotestsum's default (pkgname) instead of the testname
format set by the Makefile. Pass --format "${GOTESTSUM_FORMAT:-testname}"
to match the Makefile default.

Co-authored-by: Mattermost Build <build@mattermost.com>

server/scripts/run-shard-tests.sh

@@ -50,7 +50,7 @@ run_gotestsum() {
   RUN_IDX=$((RUN_IDX + 1))
 
   GOTESTSUM_JUNITFILE="$junitfile" GOTESTSUM_JSONFILE="$jsonfile" \
-    "$GOBIN/gotestsum" --rerun-fails=3 --packages="$1" \
+    "$GOBIN/gotestsum" --format "${GOTESTSUM_FORMAT:-testname}" --rerun-fails=3 --packages="$1" \
     -- $GOFLAGS_BASE $RACE_FLAG $coverage_flag $run_flag \
     || FAILURES=$((FAILURES + 1))
 }