fix: correctly substitute testing instructions in issue template

[jnsgruk]

Environment variable substitution in the call for testing stopped working since we factored out the testing instructions. This fix ensures that the template gets the various sections substituted out before rendering, to ensure that all the relevant fields get templated correctly.

See example output here: jnsgruk/ci-testing#51

[NucciTheBoss]

LGTM! Ben would be proud of us for using awk to render the template 😄

I think this is a good stop-gap for now to ensure that the call for testing issue template renders correctly, but I think we're starting to see the need for a utility that simplifies these operations. Better to have a swiss-army knife than shell script soup imho 😅

[techtonik]

Why inject template-testing into template.md? Maybe allow to replace default template.md with own path to template.

[NucciTheBoss]

Why inject template-testing into template.md? Maybe allow to replace default template.md with own path to template.

I think the idea is that you'll eventually be able to set a custom call for testing template. This current implementation is more a less a stop-gap for a rendering issue that I encounter in the ondemand snap: charmed-hpc/ondemand-snap#3

Ideally, if/when we switch over to a CI tool, it will be much easier for folks to just set a template. For example, if we write something in Go, we can use the text/template package. That will make it really easy for folks to set their own template.

[jnsgruk]

Well, kind of. I want to steer away from a system where every single snap has a totally different template. Consistency here is a good thing, but we do need a way to customise testing instructions specifically - a lot of the other stuff won't change between snaps :)

[techtonik]

Does Go text/template allow block overriding? Like in Jinja https://jinja.palletsprojects.com/en/3.0.x/templates/#template-inheritance

[NucciTheBoss]

Yep! Go's text/template supports inheritance! https://towardsdev.com/go-text-templates-advanced-concepts-for-dynamic-content-d9e4067e91b7

[techtonik]

Yep! Go's text/template supports inheritance! https://towardsdev.com/go-text-templates-advanced-concepts-for-dynamic-content-d9e4067e91b7

Am I right that this is for HTML only?

Is there a way to specify default values for define? Looks like child template can not inherit parent, so there is a separate Go code that calculates the values for base template.