fix README file to be more verbose match actual situation (recomitted to push into 4.1)

the currently existing test cases, simply execute ./mysql-test-run in

this directory. It will fire up the newly built mysqld and test it.

If you want to run the test with a running MySQL server use the --external

option to mysql-test-run.

If you want to run a test with a running MySQL server use the --extern

option to mysql-test-run. Please note that in this mode the test suite

expects user to specify test names to run. Otherwise it falls back to the

normal "non-extern" behaviour. The reason is that some tests

could not run with external server. Here is the sample command

to test "alias" and "analyze" tests on external server:

mysql-test-run --extern alias analyze

To match your setup you might also need to provide --socket, --user and

other relevant options.

Note that you do not have to have to do make install, and you could

actually have a co-existing MySQL installation - the tests will not