tests/zvsh_test: update tests for nexe execution
tests/zvsh_test: tests for _get_manifest_and_nvram
zvsh: support for channels with @ syntax

Files specified as arguments on the command using the @ prefix are now
mapped properly as channels in the nvram file.
tests/zvsh_test: add tests for @ file channels

When a file is specified in the command line args prefixed with a @, we
need to make sure a channel is properly mapped for it in the nvram file.

These tests address a few various cases which use the @ syntax.