test: define default clang args for testing

[jnikula]

For starters, define the language stardard to use. Use c17 and c++17, fairly arbitrarily. Tests can override this. We can remove the std option from template.yaml.

This is a bit more convoluted than should be necessary, for a few reasons:

• Extension tests use the directive options from directive string. For that, the defaults should be specified in conf.py, but hawkmoth_clang is not domain specific, and can't be used. See [1]. We need to insert the defaults in directive options.

• We want to preserve the original directive options, and present those in examples documentation. However, in all testing, we need to amend the defaults.

Note that the parser selects the -x language option based on domain and filename suffix.

[1] https://github.com/jnikula/hawkmoth/issues/255

[jnikula]

Got a better idea, dropping this one.