Generalize timeout tests. Fix twitter timeout.

[peterneyens]

I generalized the tests which check if a FetchMonadError can timeout a Query.

Before, two tests didn't actually run for both TwitterFuture and Rerunnable, because they were asynchronous and the tests stopped before the results were returned. One of these tests failed for TwitterFuture and Rerunnable.
When a Timout was set for a asynchronous Query it would always time out. Timer.Nil was used to execute the timeouts, which invokes tasks immediately. I changed it to JavaTimer which is based on java.util.Timer (which is already used in the timeout for scala.concurrent.Future).

[codecov-io]

Codecov Report

Merging #131 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
+ Coverage   78.63%   78.81%   +0.17%     
==========================================
  Files          15       15              
  Lines         323      321       -2     
  Branches        2        2              
==========================================
- Hits          254      253       -1     
+ Misses         69       68       -1

Impacted Files	Coverage Δ
twitter/jvm/src/main/scala/TwitterFuture.scala	80% <100%> (-2.36%)	⬇️
monix/shared/src/main/scala/monix.scala	78.57% <0%> (+7.14%)	⬆️

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