Update SnippetsTests

[epicgamer1234]

What this pull request does

This pull request fixes the flaky test SnippetsTests.pick_should_pick_key_pairs_corresponding_to_keys. The test passes sometimes and fails other times because the Snippets.pick method does not preserve the order of elements in a list, while the test assumes a specific order.

Why the test is flaky

The containsExactly method assumes that the map groups will have a specific order, but theSnippets.pick method can return a map with elements in any order.

How the test was fixed

The test can be fixed by replacing containsExactly with containsOnly, which does not assume a specific order.

Reproduce the test failure

Run the tests with the NonDex maven tool and use the following command:
mvn -e -pl . edu.illinois:nondex-maven-plugin:2.1.1:nondex -Dtest=snippets.SnippetsTests#pick_should_pick_key_pairs_corresponding_to_keys
Fixing the flaky test now may prevent flaky test failures in future Java versions.

Result from testing

We get the following failure:
[ERROR] pick_should_pick_key_pairs_corresponding_to_keys(snippets.SnippetsTests) Time elapsed: 0.039 s <<< FAILURE! java.lang.AssertionError: Actual and expected have the same elements but not in the same order, at index 0 actual element was: <MapEntry[key="c", value=3]> whereas expected element was: <a=1>