Defer preferred_dir validation until root_dir is set

[kevin-bates]

When one trait depends on another for its validity, both traits should be loaded prior to the validation of the dependent trait. Such a relationship occurs between preferred_dir and root_dir, where preferred_dir must reside within the hierarchy of root_dir. However, when preferred_dir is defined via a CLI parameter (e.g., --ServerApp.preferred_dir = /foo), its validation will occur prior to the finalization of root_dir if root_dir is defined in a configuration file (since CLI traits are validated prior to those loaded from config files). In such cases, we need to defer the validation of preferred_dir (relative to the value of root_dir) until root_dir is validated. This pull request detects when preferred_dir is set via the CLI and, in such cases, defers its validation until root_dir is validated OR the application's superclass is initialized. This latter scenario will occur if the default value of root_dir is used (i.e., current working directory).

In the course of testing, it turns out that there's no way to test various configuration locations of root_dir since the pytest fixtures pass this trait directly (so even the default value can't be used). This pull request introduces the ability to configure root_dir by detecting it has a None value and, in such cases, removes it from the keyword arguments.

Resolves: #806

[codecov-commenter]

Codecov Report

Merging #826 (25144f4) into main (196ee87) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #826      +/-   ##
==========================================
+ Coverage   70.09%   70.21%   +0.12%     
==========================================
  Files          63       63              
  Lines        7480     7494      +14     
  Branches     1253     1258       +5     
==========================================
+ Hits         5243     5262      +19     
+ Misses       1855     1851       -4     
+ Partials      382      381       -1     

Impacted Files	Coverage Δ
jupyter_server/pytest_plugin.py	88.18% <100.00%> (+0.10%)	⬆️
jupyter_server/serverapp.py	65.31% <100.00%> (+0.39%)	⬆️
jupyter_server/services/kernels/handlers.py	58.51% <0.00%> (+1.06%)	⬆️

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[blink1073]

LGTM, thanks!

[echarles]

@meeseeksdev please backport to 1.x