Cleanup test classes

[strangelookingnerd]

Related: apache/httpcomponents-core#470

This PR cleans up the test classes. The proposed changes include:

Remove superfluous exceptions in throws clauses
Superfluous exceptions within throws clauses have negative effects on the readability and maintainability of the code.

Test classes and methods should have default package visibility
It is recommended to use the default package visibility to improve readability:

Test classes, test methods, and lifecycle methods are not required to be public, but they must not be private.
It is generally recommended to omit the public modifier for test classes, test methods, and lifecycle methods unless there is a technical reason for doing so – for example, when a test class is extended by a test class in another package. Another technical reason for making classes and methods public is to simplify testing on the module path when using the Java Module System.
— JUnit5 User Guide

Fix assertion argument orders
To me it seems like some assertions where not correctly upgraded when JUnit5 was introduced, where the order of arguments for some Assertions methods was changed.