in an ".opt" file are defined to some value (even
if it is empty). Without this, a test suite run
aborted on Windows for "embedded".

This fix was applied dusing the build of 5.4.2-beta.


mysql-test/mysql-test-run.pl:
  From the code (function "envsubst()"), it is obvious
  that any variable which is used in an ".opt" file needs
  to be defined.
  If the value is wrong, starting the server may fail -
  but if the variable is undefined, the whole test suite
  will abort.
  
  So variables which are used in an ".opt" file should
  always be defined to some value, even if it is blank.
  
  This fix may be overkill, I did not check in detail
  whether all these variables are really used in an ".opt"
  file - but some are.