fixed flaky test in ToStringBuilderTest.java

[mumbler6]

The order of items in the map variable for the ParamStore class is indeterministic, causing the map_recurse() test case to sometimes fail. Since map is a HashMap and the items inside are unordered, the conversion to string doesn't always guarantee the same string.

This PR proposes to change the map variable from a HashMap to a LinkedHashMap, preserving the ordering of insertion and removing the flakiness of the test.

This change was confirmed by running the NonDex tool, which explores and reports errors in different behaviors of under-determined Java APIs.

To reproduce this problem, you can run the test with NonDex using this command:

mvn edu.illinois:nondex-maven-plugin:2.1.7:nondex -pl ebean-api -Dtest=io.ebean.bean.ToStringBuilderTest#map_recurse

Here are screenshots of the outputs produced by NonDex before and after the fix:

Please let me know if you'd like to discuss the fix.

[rbygrave]

Nice, thanks !!