This function is not exported from the package namespace, and must be
  invoked using triple colons, BiocGenerics:::testPackage(); it
  is provided primarily for the convenience of developers.  When invoked with missing or NULL pkgname argument, the
  function assumes that it has been invoked from within the package
  source tree (or that the source tree is located above path),
  and finds unit tests in subdir="unitTests" in either the base
  or inst/ directories at the root of the package source
  tree. This mode is useful when developing unit tests, since the
  package does not have to be re-installed to run an updated test.
  When invoked with pkgname set to the name of an installed
  package, unit tests are searched for in the installed package
  directory.