[AVRO-2943] Improve GenericData String/Utf8 MAP unit test names

Improves clarify of what the tests are actually verifying.

Follow-up to:
  apache#974

https://issues.apache.org/jira/browse/AVRO-2943

lang/java/avro/src/test/java/org/apache/avro/generic/TestGenericData.java

@@ -157,7 +157,7 @@ void testEquals() {
   }
 
   @Test
-  public void testMapKeyEquals() {
+  public void testMapKeyEqualsStringAndUtf8Compatibility() {
     Schema mapSchema = new Schema.Parser().parse("{\"type\": \"map\", \"values\": \"string\"}");
     Field myMapField = new Field("my_map", Schema.createMap(mapSchema), null, null);
     Schema schema = Schema.createRecord("my_record", "doc", "mytest", false);
@@ -178,7 +178,7 @@ public void testMapKeyEquals() {
   }
 
   @Test
-  public void testMapValuesEquals() {
+  public void testMapValuesEqualsStringAndUtf8Compatibility() {
     Schema mapSchema = new Schema.Parser().parse("{\"type\": \"map\", \"values\": \"string\"}");
     Field myMapField = new Field("my_map", Schema.createMap(mapSchema), null, null);
     Schema schema = Schema.createRecord("my_record", "doc", "mytest", false);