(feat): Add test cases for layer.ShouldProcess()

[lasith-kg]

When adding lvm support to ebs-bootstrap, I realised that all layers were being invoked, even if the relevant flag for a layer was disabled across all devices. For example, we were querying for the sizes of all block devices and their file systems, even if the resize flag was disabled across all devices.

This was not a huge issue until we added lvm. Suddenly the LVM layers were being invoked and attempting to query the system with utilities like lvs and pvs. We can not reasonably expect that these tools will be pre-installed on the operating system.

Therefore I expanded the layer interface by adding a ShouldProcess(c *config.Config) method. This method would assess the config and determine whether the layer should be invoked. This brings performance gains as now the tool can avoid the invocation of layers that are not relevant to the consumer.

Here are test cases for this new layer.ShouldProcess() method