[tests] tests for tryLock

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
test each part in a separate step

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[subprocess] fixing rc, adding start() wrapper

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[os] df/free: do not rely on btrfs tool presence

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
Merge pull request #4 from nemomobile/staging

tryLock, proper testing and small fixes