Minor improvements to analyzer test naming (#21234)

Clarify the intent of these tests.

packages/flutter_tools/test/commands/analyze_continuously_test.dart

@@ -70,10 +70,11 @@ void main() {
     OperatingSystemUtils: () => os
   });
 
-  testUsingContext('analyze', () async {
+  testUsingContext('Returns no errors when source is error-free', () async {
     const String contents = "StringBuffer bar = StringBuffer('baz');";
     tempDir.childFile('main.dart').writeAsStringSync(contents);
     server = new AnalysisServer(dartSdkPath, <String>[tempDir.path]);
+
     int errorCount = 0;
     final Future<bool> onDone = server.onAnalyzing.where((bool analyzing) => analyzing == false).first;
     server.onErrors.listen((FileAnalysisErrors errors) {

packages/flutter_tools/test/commands/analyze_once_test.dart

@@ -162,7 +162,7 @@ void bar() {
       }
     });
 
-    testUsingContext('analyze', () async {
+    testUsingContext('returns no issues when source is error-free', () async {
       const String contents = '''
 StringBuffer bar = StringBuffer('baz');
 ''';