feat(regression): add "many utxos" test option

Extend regression.sh and workflow to support running the "many utxos"
test. This adds a new MARKEXPR value and updates the workflow choices
to include "many utxos" for targeted test execution.

Author: mkoura

.github/regression.sh

@@ -49,6 +49,9 @@ if [ "$MARKEXPR" = "all" ]; then
 elif [ "$MARKEXPR" = "conway only" ]; then
   unset MARKEXPR
   export TESTS_DIR="cardano_node_tests/tests/tests_conway"
+elif [ "$MARKEXPR" = "many utxos" ]; then
+  unset MARKEXPR
+  export TESTS_DIR="cardano_node_tests/tests/test_tx_many_utxos.py"
 elif [ "$MARKEXPR" = "dbsync config" ]; then
   export CLUSTERS_COUNT=1
   export MARKEXPR="(dbsync and smoke) or dbsync_config"

.github/workflows/regression.yaml

@@ -26,6 +26,7 @@ on:
         - team_plutus
         - not long
         - conway only
+        - many utxos
         default: all
       topology:
         type: choice