for push in 5.1 (I will inform Trudy).
Storage engines and plugins can now have unit tests to test their components; such
test must be an executable C/C++ program which name ends with '-t' and which is
obeys the mytap protocol, it must be stored in the storage engine's or plugin's
source directory (storage/<engine> or plugin/<plugin>) or any subdirectories of
this.
The top-level Makefile target "test-unit" will run all unit tests: it will scan
the engines' and plugins' directories, recursively, and execute all executable
files which name ends with '-t'."


Makefile.am:
  "unittest" directory must be built before "storage" and "plugin"
  because the unit tests in these directories may need libmytap.a
  which is in unittest/mytap.
config/ac-macros/plugins.m4:
  When enabling engine "X", we add "../storage/X" to the unit tests
  directories which unittest/unit.pl should traverse looking for
  tests to execute. Same for plugins.
unittest/Makefile.am:
  Those variables contain all enabled engines and plugins.