Print an error if no tests match in runner.

Otherwise it's confusing if you mistype the test name.

Change-Id: Idf32081958f85f3b5aeb8993a07f6975c27644f8
Reviewed-on: https://boringssl-review.googlesource.com/7500
Reviewed-by: Emily Stark (Dunn) <estark@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

ssl/test/runner/runner.go

@@ -5087,11 +5087,17 @@ func main() {
 		go worker(statusChan, testChan, *shimPath, &wg)
 	}
 
+	var foundTest bool
 	for i := range testCases {
 		if len(*testToRun) == 0 || *testToRun == testCases[i].name {
+			foundTest = true
 			testChan <- &testCases[i]
 		}
 	}
+	if !foundTest {
+		fmt.Fprintf(os.Stderr, "No test named '%s'\n", *testToRun)
+		os.Exit(1)
+	}
 
 	close(testChan)
 	wg.Wait()