Rename TestCase to MonologTestCase

[staabm]

closes #1951

[staabm]

instead of a class_alias I went with a regular class and marked it as @deprecated.
that way consumers of this lib - or other projects which mistakenly used the old base-class - will see phpstorm/phpstan reports about the wrongly used class-name (when using class-alias the old class-name cannot be marked as deprecated)

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the failling build looks unrelated.. it errored - at least on my machine - even before this PR

-> build fixes in #1954

[Seldaek]

Thanks