tests: move tests to tests/functional

Signed-off-by: Kai Zhang <kyle@zelin.io>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
build: add 'make coverage' target

Signed-off-by: Kai Zhang <kyle@zelin.io>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
unittest: add unittest for collie/common.c

Signed-off-by: Kai Zhang <kyle@zelin.io>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
unittest: add unittest for sheep/vdi.c

Signed-off-by: Kai Zhang <kyle@zelin.io>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>