FIX: wrong library path inclusion

Without this patch this test suite emits error
with the script below.
The problem seems to be the library path inclusion.

```
rake install
dmtest list --suite bcache
dmtest list --suite enhance_io
dmtest list --suite fake-discard
dmtest list --suite infrastructure
```

Most of the fixes is to rewrite "tests" -> "dmtest/tests"

and Other fixes are
- add activerecord to the dependency
- add dmtest/device-mapper/dm.rb for facade