Apply RootPathnameMethods linting suggestion

[Splines]

Linting failed due to Rails/RootPathnameMethods, this PR fixes it.

Safety: This cop is unsafe for autocorrection because Dir's children, each_child, entries, and glob methods return string element, but these methods of Pathname return Pathname element.

Apparently, require probably also works on Pathname. At least, the tests still work for me locally. I haven't checked the API for require though. This file is only used for tests anyways and as they still work I think it's not a big deal.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.45%. Comparing base (73b0fc8) to head (80b889d).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #695   +/-   ##
=======================================
  Coverage   54.45%   54.45%           
=======================================
  Files         160      160           
  Lines        6751     6751           
=======================================
  Hits         3676     3676           
  Misses       3075     3075           

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