test: fix assertion order in session tests

[ricellis]

PR summary

Some minor clean ups in the session tests.

Associated with s681 and #599

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes - test cleanup
• Documentation content changes
• Other (please describe)

What is the current behavior?

When reviewing #599 I noticed that some of the test assertion expected/actual were reversed. This gave confusing messages when tests failed.

What is the new behavior?

• Correct the assertion method arg order to give correct messages
• Remove unused import
• MockWebServer is a Closeable so we can use try-with-resources for a cleaner test

Does this PR introduce a breaking change?

• Yes
• No

Other information