Improve unit test coverage of services repository

[yogeshbdeshpande]

Current test coverage is only 57%. Aim is to achieve >= 80%

[thomas-fossati]

I am not sure where the 80% figure comes from, but it seems rather arbitrary to me: once a minimal threshold is reached (60%?) we should aim at good tests rather than abstract coverage figures.

Anyway, as a data point, the packages falling below the 80% threshold as of today 08/15/2024 are:

	github.com/veraison/services/config: 72.4%
	github.com/veraison/services/log: 63.9%
	github.com/veraison/services/policy: 75.9%
	github.com/veraison/services/scheme/arm-cca: 71.6%
	github.com/veraison/services/scheme/parsec-cca: 78.5%
	github.com/veraison/services/scheme/psa-iot: 77.2%
	github.com/veraison/services/scheme/riot: 63.7%
	github.com/veraison/services/scheme/tpm-enacttrust: 71.3%

Note that the coverage is at least 63.7%, which means it increased by nearly 7 points since this issue was created, which is a good sign.

We also have #252 to look at currently excluded sub-packages.

[setrofim]

It should also be noted that the stats we're looking at right now are per-subpackage unit tests. That is not always meaningful. It might be useful to get a picture of our actual, overall coverage of the code base.
This would involve a couple of things:

• Getting absolude code path covereage counts (perhaps by parsing coveprofile's?), rather than percentanges, so that we can meaningfully combine them.
• Including integration tests in coverage analysis, by running them with intrumented binaries (see https://go.dev/doc/build-cover#running).