test.sh: improve test selection message

Improve how tests selection is announced by the test script.

Previously:
$ tools/test.sh -m summ_ai_sanity -vv tests/summ_ai_sanity/
[...]
Running all tests in with summ_ai_sanity, tests/summ_ai_sanity/...

Now:
$ tools/test.sh -m summ_ai_sanity -vv tests/summ_ai_sanity/
[...]
Running all tests with summ_ai_sanity in "tests/summ_ai_sanity/"...

tools/test.sh

@@ -88,15 +88,16 @@ if [[ -z "${CHANGED}" ]] && (( ${#TESTS[@]} == 0 )); then
 fi
 
 if [[ -n "${KW_EXPR}" ]] || [[ -n "${MARKER}" ]] || (( ${#TESTS[@]} )); then
-    MESSAGES=()
+    TEST_MESSAGE=""
     if [[ -n "${KW_EXPR}" ]]; then
-        MESSAGES+=("\"${KW_EXPR}\"")
+        TEST_MESSAGE+=" \"${KW_EXPR}\""
         PYTEST_ARGS+=("-k" "${KW_EXPR}")
     fi
     if [[ -n "${MARKER}" ]]; then
-        MESSAGES+=("with ${MARKER}")
+        TEST_MESSAGE+=" with ${MARKER}"
         PYTEST_ARGS+=("-m" "${MARKER}")
     fi
+    MESSAGES=()
     # Check whether test paths exist
     for t in "${TESTS[@]}"; do
         if [[ -e "${t%%::*}" ]]; then
@@ -109,8 +110,8 @@ if [[ -n "${KW_EXPR}" ]] || [[ -n "${MARKER}" ]] || (( ${#TESTS[@]} )); then
             exit 1
         fi
     done
-    TEST_MESSAGE=$(join_by ", " "${MESSAGES[@]}")
-    TEST_MESSAGE=" in ${TEST_MESSAGE}"
+    MESSAGES=$(join_by ", " "${MESSAGES[@]}")
+    TEST_MESSAGE+=" in ${MESSAGES}"
 fi
 
 "$(dirname "${BASH_SOURCE[0]}")/prune_pdf_cache.sh"