get test to run self contained in meson's test suit

[kjmeagher]

this adds detection of envvar NUFLUX_DATA to the library code also add test which reads the c++ code in the readme to make sure it is correct

[codecov[bot]]

Codecov Report

Patch coverage: 25.00% and project coverage change: +0.71 :tada:

Comparison is base (172eb73) 52.66% compared to head (8624364) 53.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #19 +/- ## ========================================== + Coverage 52.66% 53.38% +0.71% ========================================== Files 15 15 Lines 750 753 +3 Branches 328 330 +2 ========================================== + Hits 395 402 +7 + Misses 184 169 -15 - Partials 171 182 +11 ``` | [Impacted Files](https://app.codecov.io/gh/icecube/nuflux/pull/19?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=icecube) | Coverage Δ | | |---|---|---| | [src/library/detail.cpp](https://app.codecov.io/gh/icecube/nuflux/pull/19?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=icecube#diff-c3JjL2xpYnJhcnkvZGV0YWlsLmNwcA==) | `28.57% <25.00%> (-71.43%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/icecube/nuflux/pull/19/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=icecube)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.