Some distributions (notably, debian bookworm) have deprecated the
`python` interpreter in favor of `python3`. Since our scripts are
python3 anyway, use the proper numbered version in shebang to make
scripts easily executable.
This allows Gitlab to show nice summary for individual tests/test
directories and to expose the results in Gitlab API for consumption
elsewhere.
A catch: As of Gitlab 14.7.7, the detailed results are stored
only in artifacts and thus expire. All consumers (including API) need
to be "fast enough" to get the data before they disappear.
This also forces us to always store the artifacts intead of storing them
only on failure.
