Add descriptive messages for `assert` statements in tests

[aik099]

I think, that time passed from discovering, that a test have failed to the time, when we know why it failed should be as small as possible.

To ensure that I'm proposing to fill $message parameter value in each of used assert* statement in tests. This will be especially useful for tests, that contain multiple assert statements, like testAdvancedForm.

Example of bad default error message: "Failed asserting, that false is true".

P.S.
We can look at format of assert messages in Symfony test suite.

[stof]

we should probably also split such big tests into smaller ones, with a single purpose

[aik099]

we should probably also split such big tests into smaller ones, with a single purpose

Sometimes, like with testAdvancedForm, such split is a counterproductive, since a lot of common code would exist in each test. For me tests are as important as a code, that they are testing. There can't be big mess there just because there are tests. Maybe a single large GeneralDriverTest should be split into smaller test case classes as well.