ErasureCode: complete plugin loader unit tests

ErasureCode: complete plugin loader unit tests

* TestErasureCodePluginExample.cc is renamed to TestErasureCodePlugin.cc
  because it's not limited to the example which is really used to
  support tests rather than being tested.

* Bugous plugins are added to exhibit failures and enable the unit tests
  to check they are handled as expected

  ErasureCodePluginFailToInitialize : the entry point returns != 0
  ErasureCodePluginFailToRegister : the plugin registry is not updated
  ErasureCodePluginMissingEntryPoint : the shared library has no entry
  point

* It would be difficult to prove that the mutex protecting against
  multiple loads actually does what it is expected to because of the
  lack of thread introspection functions such as : tell me if this
  thread is waiting on this mutex. A simpler approach is chosen : create
  a thread that blocks forever when loading ( that's what the delay in
  the example plugin is for ) and then check that the lock has indeed
  been acquired. Since this mutex is merely about making sure that only
  one thread at a time runs this sequence of code, it's probably enough.

Signed-off-by: Loic Dachary <loic@dachary.org>