New functional tests

Following yesterday's PR:

* functional tests bootstrap improvements
* more functional tests
* a reset-database command has been added (recreates the pouch database)

## Failing tests
**All the tests** that run a `mv` don't pass, I `.skip` them so the rest can pass. The issue I noticed is that the binary is lost on the remote after those command are executed. @Kloadut told me he fixed so it's going to be a good test.

Please note that the tests following the `.skip` tests must be changed a bit (the file name) if those tests are un `.skip`. (see 
* https://github.com/jsilvestre/cozy-data-proxy/compare/cozy-labs:master...master#diff-a4c46cfa99b0cca18cc25fd45698910dR152
* https://github.com/jsilvestre/cozy-data-proxy/compare/cozy-labs:master...master#diff-a4c46cfa99b0cca18cc25fd45698910dR176
)

## Instability
It happens that sometimes the tests fail for no apparent reason. Try to re-run them again if that's the case (I've investigated it and haven't found any reason)
Tests should be run with `npm test` so it cleans the database before running the tests (haven't achieved to do it within the tests)