autotest: Make ogr gpkg tests independent, parallel-safe

[dbaston]

What does this PR do?

Adds gdaltest.reopen helper
Makes ogr gpkg tests independent, parallel-safe

What are related issues/pull requests?

#4407

Tasklist

• Review
• Adjust for comments
• All CI builds and checks have passed

[coveralls]

coverage: 67.674% (+0.001%) from 67.673% when pulling 5037410 on dbaston:pytest-independent into bda1424 on OSGeo:master.

[rouault]

There seem to be a very minor coverage regression at https://coveralls.io/builds/62971018/source?filename=ogr%2Fogrsf_frmts%2Fgpkg%2Fogrgeopackagetablelayer.cpp#L4093 . So a situation where we would call CreateSpatialIndex() on a layer that has no geometry column. That is triggered in master in test_ogr_gpkg_15() with ExecuteSQL("SELECT CreateSpatialIndex('non_spatial', '')")

[dbaston]

There seem to be a very minor coverage regression at https://coveralls.io/builds/62971018/source?filename=ogr%2Fogrsf_frmts%2Fgpkg%2Fogrgeopackagetablelayer.cpp#L4093 . So a situation where we would call CreateSpatialIndex() on a layer that has no geometry column. That is triggered in master in test_ogr_gpkg_15() with ExecuteSQL("SELECT CreateSpatialIndex('non_spatial', '')")

Restored: https://coveralls.io/builds/62973505/source?filename=ogr%2Fogrsf_frmts%2Fgpkg%2Fogrgeopackagetablelayer.cpp#L4093