WL#3504: plugin actions for engines' and plugins' unit tests

The work on WL#3234 "maria control file manager" led to creating unit tests for
Maria, and so some plugin macro was modified to enable include those unit tests
in the list of unit tests run by the top-level "make test-unit".
It has been decided by Brian to port this modification to MySQL 5.1.
Brian and Serg have reviewed the patch written by Guilhem, and it has been
pushed today in the replication team's 5.1 tree.

Documentation could read like this:
"Storage engines and plugins can 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/ or plugin/) or any subdirectories of
this.
An example of such tests can be found in the "unitttest/examples" directory of
the distribution of MySQL.
The code for the mytap protocol can be found in the "unittest/mytap" directory
of the distribution of MySQL.
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'."

Above, the distribution is very vague: it at least includes the bitkeeper tree,
the Unix source distribution made by "make dist" (is this how we make it?). As
for binary distributions or the Windows source, this has to be investigated.
Unit tests are not compiled on Windows so far, anyway.

Minor test-related task, no LLD needed.
-- Trudy Pelzer, 2006-09-15