os: tests: optimize fsck tests
os: tests: optimize fsck tests

Chain operations using Promise.then(), and run commands in parallel
using Promise.map(). This reduces the time taken for fsck tests to about
half.

- ok 1 - fsck.ext4 tests # time=152304.735ms
+ ok 1 - fsck.ext4 tests # time=75104.016ms

Change-type: patch
Signed-off-by: Joseph Kogut <461476587780aa9fa5611ea6dc3912c146a91760@balena.io>
resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin