feat(parallel): implement test-level parallelism for improved performance

Enable parallel execution of individual test functions within files,
not just parallel file execution. This significantly improves CPU
utilization and reduces test execution time.

Implementation:
- Modified call_test_functions to run test functions in parallel
- Added opt-out mechanism via `# bashunit: no-parallel-tests` marker
- Tests within a file now execute concurrently using background jobs
- Result aggregation already supports per-test result files

Opt-out marker added to tests with shared state:
- install_test.sh (shares ./lib and ./deps directories)
- bashunit_report_html_test.sh (shares HTML output files)
- bashunit_upgrade_test.sh (shares tmp/ directory)

Performance improvement:
- Parallel execution: 31.71s (down from ~40s, ~21% faster)
- CPU utilization: 702% (up from ~480%)
- Better parallelization across all available cores

All 750 tests passing, all 1136 assertions correct.

Author: Chemaclass

src/runner.sh

@@ -342,6 +342,12 @@ function bashunit::runner::call_test_functions() {
 
   bashunit::helper::check_duplicate_functions "$script" || true
 
+  # Check if test file opts out of test-level parallelism
+  local allow_test_parallel=true
+  if grep -q "^# bashunit: no-parallel-tests" "$script" 2>/dev/null; then
+    allow_test_parallel=false
+  fi
+
   local -a provider_data=()
   local provider_data_count=0
   local -a parsed_data=()
@@ -363,7 +369,11 @@ function bashunit::runner::call_test_functions() {
 
     # No data provider found
     if [[ "$provider_data_count" -eq 0 ]]; then
-      bashunit::runner::run_test "$script" "$fn_name"
+      if bashunit::parallel::is_enabled && [[ "$allow_test_parallel" == true ]]; then
+        bashunit::runner::run_test "$script" "$fn_name" &
+      else
+        bashunit::runner::run_test "$script" "$fn_name"
+      fi
       unset -v fn_name
       continue
     fi
@@ -379,10 +389,19 @@ function bashunit::runner::call_test_functions() {
         parsed_data[parsed_data_count]="$(bashunit::helper::decode_base64 "${line}")"
         parsed_data_count=$((parsed_data_count + 1))
       done <<<"$(bashunit::runner::parse_data_provider_args "$data")"
-      bashunit::runner::run_test "$script" "$fn_name" ${parsed_data+"${parsed_data[@]}"}
+      if bashunit::parallel::is_enabled && [[ "$allow_test_parallel" == true ]]; then
+        bashunit::runner::run_test "$script" "$fn_name" ${parsed_data+"${parsed_data[@]}"} &
+      else
+        bashunit::runner::run_test "$script" "$fn_name" ${parsed_data+"${parsed_data[@]}"}
+      fi
     done
     unset -v fn_name
   done
+
+  # Wait for all parallel tests within this file to complete
+  if bashunit::parallel::is_enabled && [[ "$allow_test_parallel" == true ]]; then
+    wait
+  fi
 }
 
 function bashunit::runner::call_bench_functions() {

tests/acceptance/bashunit_report_html_test.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# bashunit: no-parallel-tests
 set -euo pipefail
 
 function set_up_before_script() {

tests/acceptance/bashunit_upgrade_test.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# bashunit: no-parallel-tests
 set -uo pipefail
 set +e
 

tests/acceptance/install_test.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# bashunit: no-parallel-tests
 # shellcheck disable=SC2317
 set -uo pipefail
 set +e