Changes

testenv: add proper [testsuite] prepare= support

Support using prepare= in the [testsuite] section to e.g. change module
parameters:

  [testsuite]
  prepare=sed -i 's/mp_my_parameter := false/mp_my_parameter := true/' Testsuite.cfg

Move the execution time of prepare and clean scripts in
testdir.prepare() after the testsuite config has been copied to the
testdir, so it can be modified by the prepare script.

Change-Id: Ia78a4eba061e5e070f156f75eae1dfa4cede1ee3