probes: refactor fact snippet mixin

[leondz]

requires #1181

Refactor snippet code in latentinjection probes to move function up into FactSnippetMixin

validation:

• all probes inheriting FactSnippetMixin contain exactly one INJECTION_MARKER
• new test for FactSnippetMixin

todo (may be out of scope):

• identify a generalisable a route for mixins to make config items available through DEFAULT_PARAMS

[erickgalinkin]

This looks good to me. I think it's a really good improvement on the existing logic -- thanks!

[leondz]

Don't merge til after 0.11.0, alters bag probes content slightly

[jmartin-tech]

Test failures are due to import changes from main.

[leondz]

Thanks

Will work out a better pattern for configuration before merging. Maybe by also doing merge w Probe.DEFAULT_PARAMS and requiring appropriate mro in inheritance descriptions.