nu-cli: added tests for file completions (#5232)

* nu-cli: added tests for file completions

* test adding extra sort

* Feature/refactor completion options (#5228)

* Copy completion filter to custom completions

* Remove filter function from completer

This function was a no-op for FileCompletion and CommandCompletion.
Flag- and VariableCompletion just filters with `starts_with` which
happens in both completers anyway and should therefore also be a no-op.
The remaining use case in CustomCompletion was moved into the
CustomCompletion source file.

Filtering should probably happen immediately while fetching completions
to avoid unnecessary memory allocations.

* Add get_sort_by() to Completer trait

* Remove CompletionOptions from Completer::fetch()

* Fix clippy lints

* Apply Completer changes to DotNuCompletion

* add os to $nu based on rust's understanding (#5243)

* add os to $nu based on rust's understanding

* add a few more constants

Co-authored-by: Richard <Tropid@users.noreply.github.com>
Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

crates/nu-test-support/src/fs.rs

@@ -258,7 +258,7 @@ pub fn binaries() -> PathBuf {
 }
 
 pub fn fixtures() -> PathBuf {
-    root().join("tests/fixtures")
+    root().join("tests").join("fixtures")
 }
 
 pub fn assets() -> PathBuf {