fix flaky EnumTest

[219sansim]

Purpose

The test testClassWithEnumFieldSerialization in EnumTest is detected flaky because the gson.toJson() gives a non-deterministic output.
Related issue: #2520

Description

After modifying the tests to check if the gson.toJson() output to is one of the possible json string values, the test passes, instead of checking against only 1 string.

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

gson/gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java

Line 265 in e685705

Field[] fields = raw.getDeclaredFields();

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

Reproduction of error

The test runs fail on 4/5 runs of the NonDex tool

Command to reproduce the failure:

mvn -pl gson edu.illinois:nondex-maven-plugin:2.1.1:debug -Dtest=com.google.gson.functional.EnumTest#testClassWithEnumFieldSerialization

Error Message:

Running com.google.gson.functional.EnumTest
[ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.171 s <<< FAILURE! -- in com.google.gson.functional.EnumTest
[ERROR] com.google.gson.functional.EnumTest.testClassWithEnumFieldSerialization -- Time elapsed: 0.163 s <<< FAILURE!
value of: toJson(...)
expected: {"value1":"VALUE1","value2":"VALUE2"}
but was : {"value2":"VALUE2","value1":"VALUE1"}
        at com.google.gson.functional.EnumTest.testClassWithEnumFieldSerialization(EnumTest.java:96)

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