interfaces: ensure that legacy interface methods are unused

[zyga]

The snapd tree has seen several iteration of the interface code. Each
iteration brought changes to how interfaces are written. Up until now
interfaces were strongly typed, with each interface method being defined
in the Interface go interface. For a few months though they became
loosely typed as each security back-end detects and uses a subset of
methods that are specific to itself.

As another change is approaching, the interface methods will change
again. The way some of the unit tests are written may allow tests to
pass as the existing method are tested. At runtime those methods would
not be used though, and this is certainly undesired.

To avoid potential issues this patch adds a new unit test that looks at
all the known interfaces and makes sure that none of them define any of
the unused methods. This way each API transition can be accompanied by a
simple test extension. This should ensure that no merged pull request
(especially an older one) accidentally brings stale APIs into the tree.

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[stolowski]

That's super-useful to have, thank you! One minor nitpick, please address before merging. Approving right away.

[stolowski]

Micro-optization: break once you found a matching one ;)

[zyga]

Done :)

[mvo5]

Looks good