Allow running all test suites against locally built platform snap

[farshidtz]

The LOCAL_SNAP environment variable is used for platform or the service snap, depending on the testing suite. The only way to run a device/app/support test against a particular platform snap revision is to use the revision number or branch from the store. This requires first uploading a locally built snap which is very resource consuming.

Example:

PLATFORM_CHANNEL=4259 go test ./test/suites/device-virtual

PLATFORM_CHANNEL=latest/edge/temp-branch go test ./test/suites/device-virtual

The requirement is to allow passing a local snap file path for running any testing suite.