TestSuite: exit with proper code on test failure in Emscripten.

std::exit(1) doesn't cause Node.js to exit with non-zero code, thus all
the tests are marked as passed in CTest. To fix the issue,
CORRADE_TEST_MAIN() is calling std::abort() after proper cleanup. This
however causes ugly stack trace to be printed on output. Related to
kripken/emscripten#1371, didn't find better solution yet.