Fix flaky tests in FieldNamingTest

[219sansim]

Purpose

The tests in FieldNamingTest are detected flaky because the gson.toJson() gives a non-deterministic output.
Related issue: #2520

Description

There is a need to modify the code such that the tests to do not expect the string to contain values in a particular order. After modifying the tests to check if the gson.toJson() output contains all expected substrings, the tests pass, instead of checking against the entire hardcoded 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:nondex -Dtest=com.google.gson.functional.FieldNamingTest#<testname>

Example - test case 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)

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