WIP: restructure tests

[jvican]

• This change is useful for two reasons: it gives us a more consistent
  structure of the tests that allow fast browsing through the test suite
  and it also allow us to distinguish which tests are missing (e.g. tests
  that check the binary format but not the json format). Also, an important
  point is that this is the first step to port the test suite to Scalajs and
  reuse most of its components. For doing that, we're importing staticOnly
  all over the place and therefore we figure out which tests are Scalajs friendly
  and which are not.

Disclaimer: the Scalajs restructure will be also done in this PR.

Changes

Add staticOnly flag to most of binary tests

• Binary tests are now compiled with the static flag, however there are
  still some bad boys that need to be moved to the JVM folder, since they
  need reflection somehow.
• Rename a rebel test filename to follow current nomenclature.

Group better the tests

• Rename logic to generation to store all the tests related with the
  correct generation of picklers for any type.
• Move generator, pickler, tags and internal to generation.

Rename tests to specify their own format

• When the test suite is run, it's confusing to know figure out the failed
  test since some tests are repeated for different formats
  (binary and json) with the same test name.

Merge the tests and change the overall structure

• This change is useful for two reasons: it gives us a more consistent
  structure of the tests that allow fast browsing through the test suite
  and it also allow us to distinguish which tests are missing (e.g. tests
  that check the binary format but not the json format).
• Also, it removes ~3 repeated tests and it adds ~3 test suites. One of
  them checks that our picklers are thread-safe in both formats.
• Some renames have been made to make clearer what tests are supposed to
  do, and therefore they are a little bit more verbose. A little subset
  has the name of the test class (and not only the file) changed, but
  this is practically unimportant.
• Lastly, the modularity of the tests will help to have more granularity
  to select tests for the scalajs support. That is the next step.

Update scalatest and fix scalatest issue 873

• Update to M15 of latest release of scalatest that is now ScalaJS
  compatible.
• Make sure sbt recognises the dependencies as test runners.
• Fix scalatest issue 873, caused by a bad interaction between the
  assert macro and our generated picklers and unpicklers. The assert
  macro is going to cache the condition that is testing and is going to
  convert it to a String to show it to the user in case of failure.
  If the condition that is evaluated by assert uses pickle and
  unpickle and the generated picklers/unpicklers are very big, they
  surpass the 65k length limit imposed by the code generators (both in
  ScalaJS and the JVM) since they store the length in two bytes.

[jsuereth]

I like these distinctions, with the caveat that perhaps these should be called 'regression tests' to prevent bugs from reopening.

[jsuereth]

Looks great!