11.8.0: Testing migration

Heads up!

• Merged PR #331: Migrate to new testing library. This makes it easier
  to run the test suite (now just xp test src/test/php like with any
  other library) and reduces the installed size by roughly 300 kB.
  Preliminary work was put into this to make the transition easier:
  • PR #330: Use COMPOSER_ROOT_VERSION and add dependency instead of
    bundling test library
  • PR #329: Restructure src/test/php - from tests inside the package
    net.xp_framework.unittest to the package (lang, util, io, xp)
    they're testing
  • Merged PR #328: Migrate tests to baseless / Assert DSL
    (@thekid)