LU-8191 tests: convert functions to static

LU-8191 tests: convert functions to static

Static analysis shows that a number of functions
could be made static. This patch declares several
functions in various test helpers static.

Lustre-change: https://review.whamcloud.com/51433
Lustre-commit: 629d6bca95f96bd307d6a9da9b04d73d3fe7c68f

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I065fb4398ed1670ce6ad58cf946054f6bd1ec282
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54603
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>