Finding test db location from tests

[Vladimir Prus]

I've posted this question some time ago, but nothing was decided, so I'm 
trying again. I would like to be able to find the location of test db in
test's 'run' method. I need it in order to appropriately set search 
paths, find binaries in the working copy, etc. That is, I want to test 
binary located in $(tdb_location)/../bin, etc.

The only way now, is to pass the location via context. This has the 
problem that you need to create that context file, and the content is 
different for different working copies. (I cannot use relative paths 
there, because I don't know the base path for them). This is inconvenient.

I think, the only change needed is to allow test classes find the test 
db used -- test db has 'GetRoot' method already. Is this possible?

TIA,
Volodya