feat(tvix/glue): add some import tests

This creates a directory with a .keep file inside, and uses
"${path/to/there}" to coerce it to a string (and import it into the
store), ensuring it calculates the same store paths as Nix does.

Change-Id: Ie14ae075104ce278bc4f2cce93aab5762a2734d1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10343
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/glue/Cargo.toml

@@ -20,6 +20,7 @@ git = "https://github.com/tvlfyi/wu-manber.git"
 [dev-dependencies]
 criterion = { version = "0.5", features = ["html_reports"] }
 lazy_static = "1.4.0"
+tempfile = "3.8.1"
 test-case = "2.2.2"
 
 [[bench]]