fileserver: fix try_policy when instantiating file matcher from CEL

[effleurager]

I found a minor bug in #6623 for the file matcher when written as a CEL expression.

This PR corrects the current reassignment of root, and adds a test to ensure the correct property is set.

[francislavoie]

Thanks!

I'm not sure that test properly covers it though? Wouldn't it need more than one try_files at least to tell them apart with that policy? I figure it could be tricky tho because git might make those test files have the same mod time though 🤔 not sure how to best test that.

[francislavoie]

I reworked the test to use largest_size which we can guarantee when using Git (since file mod time is based on when you checkout the changes etc), and validating that it does indeed pick the correct file by checking the value in the replacer.

[mholt]

Thanks both!