Test suite and test workflow

[lars-t-hansen]

I didn't want to make this part of #172 since it's broader. Currently:

• the github workflow runs cargo build but then each test case in tests/ also does that; the latter are redundant
• with that in mind, the building of the C test helpers in the test dir should be done once by the workflow probably
• we may want a toplevel build.sh script that does all of that for us, for convenience when developing
• we want test cases to test the output format of sonar ps better. right now there's some ad-hoccery around some of the fields, we should test more fields, in particular timestamp (cf #173, currently an internal test only)
• (more ideas here)

[lars-t-hansen]

More ideas:

• an acceptance criterion for release is to pass local testing on hardware of every GPU type, which means running it manually on these systems
• ideally we would have some automation to run testing daily on hardware of every GPU type and report problems by email or other push notification