Regenerate ROOT data for celeritas tests if missing

[sethrj]

I propose this as a simpler alternative to #1516. As the reviewers previously described on slack, running

$ ninja clean
$ ninja
$ ctest

fails because the ROOT files are not regenerated.

With this, ROOT data is regenerated automatically as part of celeritas/test/all (or specifically, the testcel_celeritas library) if are absent when building. Importantly, running ninja doesn't renew the files if they already exist.

[stognini]

Tried a fully clean build here. The files existed, but were overridden during the fresh build, leading to

❯ git st
 M ../test/celeritas/data/four-steel-slabs.root
 M ../test/celeritas/data/lar-sphere.root
 M ../test/celeritas/data/simple-cms.root

[github-actions]

Test summary

 3 844 files   5 936 suites   4m 8s ⏱️
 1 597 tests  1 564 ✅ 28 💤 5 ❌
19 790 runs  19 708 ✅ 75 💤 7 ❌

For more details on these failures, see this check.

Results for commit c84f224.

[sethrj]

Ooh that's not what we want either...

[sethrj]

This appears to be happening (and on the CI) because of the many dependencies on the "custom command". Rebuilding the celer-export-geant binary forces the file to become out of date, and cloning/updating some of the other files does the same. This is technically correct behavior, but in practice we don't want to regenerate the files except rarely. I'm not sure what the best thing to do is; maybe #1516 really is the way to go.