[PATCH 0 of 3] some changes for run-test (resend 2)

[Simon Heimberg]

#1 and #2 were sent earlyer, but messed up and with more changes
#3 is a new idea which (possibly) removes a to do comment after several 1000 changesets.

with #1, runtest --with-hg=anything does not fail anymore. But its value is 
ignored. (The global variables BINDIR and PYTHONDIR are set, but they are 
only used in code which is executed when installing (and in coverage, 
but this can not be used because the coverage script is only written on 
installation)).
The only possibility to change the called hg binary is the environment 
variable PATH, because hg is called in the test scipts. What mercurial 
module hg uses can be influenced by PYTHONPATH. But this 
environment variables are not set.