[tests] Fix PATH use in tests.xml

Setting environment variables in <pre_steps /> has not effect - it is
evaluated in separate shell.

Relates NEMO#520
[tests] Do not hardcode variable paths

Relates NEMO#520
[tests] Fix lca-cita-test file permissions (+x)

Relates NEMO#520
[tests] testlib.sh: improve error reporting

Relates NEMO#520
[tests] test-actions: fix permission denied error

Relates NEMO#520
[tests] Allow to execute tests on target

Originally it was supposed tests would be executed inside build dir.

Notice that env.py.in and env.sh.in have not been left unused after f79e5b1 - removing now.

Relates NEMO#520