[Chapel Merge] Use python3 in a few test scripts rather than pyth

Branch: refs/heads/master
Revision: 5fa6e87
Author: mppf
Log Message:

Merge pull request #16624 from mppf/testing-use-python3

Use python3 in a few test scripts rather than python

Follow-up to PR #16560

Since python is not available on many current linux distributions
and since python3 is the current testing dependency.

Test change only - not reviewed.

Modified Files:
M test/library/standard/FileSystem/fsouza/chown/chown_nothing.prediff
M test/library/standard/FileSystem/fsouza/chown/chown_nothing.preexec
M test/performance/compiler/elliot/updateDatFiles.chpl
M test/setchplenv/sub_test
M test/studies/amr/advection/amr/AMR_AdvectionCTU_driver.prediff
M test/studies/amr/advection/grid/Grid_AdvectionCTU_driver.prediff
M test/studies/amr/diffusion/grid/Grid_DiffusionBE_driver.prediff
M test/studies/amr/diffusion/level/Level_DiffusionBE_driver.prediff

Compare: https://github.com/chapel-lang/chapel/compare/cfc9a13754a0...5fa6e871a2d6