Fix flaky declared fields method

[219sansim]

Purpose

This PR fixes the following tests detected as flaky

com.google.gson.functional.CustomTypeAdaptersTest.testCustomNestedSerializers
com.google.gson.functional.EnumTest.testClassWithEnumFieldSerialization
com.google.gson.functional.FieldNamingTest.testIdentity
com.google.gson.functional.FieldNamingTest.testLowerCaseWithDashes
com.google.gson.functional.FieldNamingTest.testLowerCaseWithUnderscores
com.google.gson.functional.FieldNamingTest.testUpperCamelCase
com.google.gson.functional.FieldNamingTest.testUpperCamelCaseWithSpaces
com.google.gson.functional.FieldNamingTest.testUpperCaseWithUnderscores
com.google.gson.functional.JsonAdapterAnnotationOnFieldsTest.testExcludeDeserializePrecedence
com.google.gson.functional.JsonAdapterSerializerDeserializerTest.testJsonSerializerDeserializerBasedJsonAdapterOnFields
com.google.gson.functional.MapAsArrayTypeAdapterTest.testMapWithTypeVariableSerialization
com.google.gson.functional.MapAsArrayTypeAdapterTest.testMultipleEnableComplexKeyRegistrationHasNoEffect
com.google.gson.functional.MapAsArrayTypeAdapterTest.testSerializeComplexMapWithTypeAdapter
com.google.gson.functional.MapTest.testInterfaceTypeMap
com.google.gson.functional.MapTest.testInterfaceTypeMapWithSerializer
com.google.gson.functional.NamingPolicyTest.testGsonDuplicateNameDueToBadNamingPolicy
com.google.gson.functional.NamingPolicyTest.testGsonWithSerializedNameFieldNamingPolicySerialization
com.google.gson.functional.ObjectTest.testArrayOfArraysSerialization
com.google.gson.functional.ObjectTest.testArrayOfObjectsSerialization
com.google.gson.functional.ObjectTest.testBagOfPrimitivesSerialization
com.google.gson.functional.ObjectTest.testBagOfPrimitiveWrappersSerialization
com.google.gson.functional.ObjectTest.testNestedSerialization
com.google.gson.functional.ObjectTest.testNullFieldsSerialization
com.google.gson.functional.PrettyPrintingTest.testEmptyMapField
com.google.gson.functional.ReadersWritersTest.testWriterForSerialization
com.google.gson.functional.ReflectionAccessFilterTest.testBlockInaccessibleJavaExtendingJdkClass
com.google.gson.functional.SerializedNameTest.testFirstNameIsChosenForSerialization
com.google.gson.functional.StreamingTypeAdaptersTest.testSerializeNullObject
com.google.gson.functional.StreamingTypeAdaptersTest.testSerializeRecursive
com.google.gson.functional.VersioningTest.testVersionedClassesSerialization
com.google.gson.functional.VersioningTest.testVersionedGsonWithUnversionedClassesSerialization
com.google.gson.graph.GraphAdapterBuilderTest.testSerialization
com.google.gson.graph.GraphAdapterBuilderTest.testSerializationWithMultipleTypes
com.google.gson.MixedStreamTest.testWriteMixedStreamed
com.google.gson.ObjectTypeAdapterTest.testSerialize
com.google.gson.typeadapters.PostConstructAdapterFactoryTest.testList

Description

This line of code does not return the fields in a consistent order causing the tests to be flaky. The fix sorts these fields using the field names as a comparator.

Reason:
The gson.toJson() internally calls com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields which makes a call to getDeclaredFields

https://github.com/google/gson/blob/e685705b2bf3ae174958612a185bd231c0e0c5d9/gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java#L265

And according to this stackoverlow discussion, getDeclaredFields in Java 8 does not return elements in a consistent order.

Reproduction of error

Command to reproduce the failure:
mvn -pl gson edu.illinois:nondex-maven-plugin:2.1.1:nondex -Dtest=<testclass>#<testname>

Example - test case FieldNamingTest.testLowerCaseWithUnderscores

mvn -pl gson edu.illinois:nondex-maven-plugin:2.1.1:nondex -Dtest=com.google.gson.functional.FieldNamingTest#testLowerCaseWithUnderscores

Error Message:

[ERROR] com.google.gson.functional.FieldNamingTest.testLowerCaseWithUnderscores -- Time elapsed: 0.003 s <<< FAILURE!
value of: replace(...)
expected: {'lower_camel':1,'upper_camel':2,'_lower_camel_leading_underscore':3,'__upper_camel_leading_underscore':4,'lower_words':5,'u_p_p_e_r__w_o_r_d_s':6,'annotatedName':7,'lower_id':8,'_9':9}
but was : {'annotatedName':7,'__upper_camel_leading_underscore':4,'lower_id':8,'upper_camel':2,'u_p_p_e_r__w_o_r_d_s':6,'lower_camel':1,'_9':9,'lower_words':5,'_lower_camel_leading_underscore':3}
	at com.google.gson.functional.FieldNamingTest.testLowerCaseWithUnderscores(FieldNamingTest.java:74)