[et] Fix concurrent modification exception

We cannot modify the list of build targets as we're iterating over it.
Instead of removing non-test/non-executable elements, instead we add
executable test targets to a separate testTargets list and use that.

tools/engine_tool/lib/src/commands/test_command.dart

@@ -70,18 +70,18 @@ et test //flutter/fml:fml_benchmarks  # Run a single test target in `//flutter/f
       );
     }
 
+    final List<BuildTarget> testTargets = <BuildTarget>[];
     for (final BuildTarget target in selectedTargets) {
-      if (!target.testOnly || target.type != BuildTargetType.executable) {
-        // Remove any targets that aren't testOnly and aren't executable.
-        selectedTargets.remove(target);
+      if (target.testOnly && target.type == BuildTargetType.executable) {
+        testTargets.add(target);
       }
       if (target.executable == null) {
         environment.logger.fatal(
             '$target is an executable but is missing the executable path');
       }
     }
     // Chop off the '//' prefix.
-    final List<String> buildTargets = selectedTargets
+    final List<String> buildTargets = testTargets
         .map<String>(
             (BuildTarget target) => target.label.substring('//'.length))
         .toList();
@@ -95,7 +95,7 @@ et test //flutter/fml:fml_benchmarks  # Run a single test target in `//flutter/f
     final WorkerPool workerPool =
         WorkerPool(environment, ProcessTaskProgressReporter(environment));
     final Set<ProcessTask> tasks = <ProcessTask>{};
-    for (final BuildTarget target in selectedTargets) {
+    for (final BuildTarget target in testTargets) {
       final List<String> commandLine = <String>[target.executable!.path];
       tasks.add(ProcessTask(
           target.label, environment, environment.engine.srcDir, commandLine));