Fix tests for env-var filtering due to previous commit

Fix tests for env-var filtering due to previous commit

The previous commit altered vkCreateInstance erroring out if the
application tries to enable an env-var disabled layer, so the tests
need to be revised for it. This commit also makes lot of modifications
to the tests in addition to that.

Also adds tests for when the override layer is present and when
implicit layer's enable-environment variables are present.