Export method  `observer.(*ObservedLogs).Filter` to allow consumers to
filter logs by arbitrary functions for testing.

Currently consumers can call `(*ObservedLogs).All` and proceed to filter
using `[]LoggedEntry` directly, but the result is then incompatible with
existing methods such as `FilterMessage` because there is now way to
re-construct an `*ObservedLogs` object.

Method `observer.(*ObservedLogs).FilterLevelExact` allows consumers to select
entries logged at a given level, for testing.