[test_runner/vm] Provide some troubleshooting info when missing vm test runner

TEST=ci and locally with missing runner

Change-Id: Ia96829097dc05bea6a683dbefba36d1d6fa96ebb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373325
Reviewed-by: Nate Bosch <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>
Reviewed-by: Ryan Macnak <[email protected]>

Author: parlough

pkg/test_runner/lib/src/test_suite.dart

@@ -280,6 +280,13 @@ class VMTestSuite extends TestSuite {
       hostRunnerPath = targetRunnerPath;
     }
 
+    if (!File(hostRunnerPath).existsSync()) {
+      throw Exception(
+          "Failed to find VM test runner binary at '$hostRunnerPath'.\n"
+          "Did you include the 'runtime' or 'most' targets in "
+          'the corresponding build?');
+    }
+
     if (configuration.useQemu) {
       final config = QemuConfig.all[configuration.architecture]!;
       initialHostArguments.insert(0, hostRunnerPath);