fix(harness): don't error out when attempting attaching storage before begin

[tonyandrewmeyer]

One minor fix to the storage metadata: charmcraft.yaml and metadata.yaml support a multiple.range value in the form <n>+ - this would previously error out when we attempted to read the metadata. We now support this (as an alias for <n>-, which is an open-ended range rather than "n or less", and have a test for it.

Adds a minor clarification in the API docs when Harness.add_storage will emit a storage-attached event.

In begin_with_initial_hooks, if the storage has already been attached, then don't try to attach it a second time, but do emit the storage-attached event, since that will not have been done previously.

Make the _storage_attach method safe to call multiple times. This appears to have been the intention based on comments in the code, but, in practice, attempting to symlink the mount would fail. Now, if the symlink already exists (and has the same target) then we continue.

Adds a variety of tests for Harness add_storage and attach_storage behaviour.

Fixes #1127

[benhoyt]

Great, thanks for this. Nit and comment about test structure.

[IronCore864]

Great test cases, I've learned a lot :)

[benhoyt]

Very clean, thanks!