Fixed bugs and added support of module-based unit tests for `mpl`.

* test/unit/jamfile:
  * Fixed bugs. See the comments for detail.
  * Added dependency on unit tests of `mpl`.
* test/unit/mpl/jamfile: Removed `compile`, `compile-fail`, `run` and
  `run-fail` targets.
* test/unit/mpl/string_compile.cpp: Moved to `test/unit/mpl/string_run.cpp`.
* test/unit/mpl/string_run.cpp: Replaced tests with module-based test cases.