Change the message sent to IntelliJ when starting the test suites to the correct one (#8330)

This is the cause of the error mentioned in #8325.
Previously, the correct message was being sent hard-coded and was
supposed to be replaced by `ServiceMessageBuilder.testsStarted()` once
the method was available. Instead,
`ServiceMessageBuilder.testStarted("")` was used which changes the
message to `##teamcity[testStarted name='']` from
`##teamcity[enteredtheMatrix]` (hence the error).

This one-liner just replaces the erroneous call to the correct one.

Author: helomri

flutter-idea/src/io/flutter/test/DartTestEventsConverterZ.java

@@ -401,7 +401,7 @@ private boolean handleStart(JsonObject obj) throws ParseException {
     mySuiteData.clear();
     mySuitCount = 0;
 
-    return doProcessServiceMessages(ServiceMessageBuilder.testStarted("").toString());
+    return doProcessServiceMessages(ServiceMessageBuilder.testsStarted().toString());
   }
 
   @SuppressWarnings("RedundantThrows")