[PHP] ObjectSerializerTest: add tests for ObjectSerializer::deserialize() associative arrays bugfix

[thomasphansen]

PR #12849 fixed a small bug in ObjectSerializer::deserialize(). As requested here by @wing328, this PR adds tests for it.

There are two commits here: the first one does some code cleanup to the test class, while the second one adds the new tests.

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[wing328]

Thanks for the PR. Tests passed via https://app.travis-ci.com/github/OpenAPITools/openapi-generator/builds/253548525