tectonic: add passthru.tests.workspace

Also, replace `builtins.toFile` with `pkgs.writeText` because the former
blocks the evaluation loop just like an import from derivation (IFD).
Although it is _technically_ not an IFD, it has very similar effects,
and thus we should prefer the nixpkgs `pkgs.writeText` instead.

pkgs/tools/typesetting/tectonic/tests.nix

@@ -3,6 +3,7 @@
 
 { lib
 , fetchFromGitHub
+, writeText
 , runCommand
 , tectonic
 , curl
@@ -26,7 +27,7 @@ let
   };
   testfiles = "${biber-dev-source}/testfiles";
 
-  noNetNotice = builtins.toFile "tectonic-offline-notice" ''
+  noNetNotice = writeText "tectonic-offline-notice" ''
     # To fetch tectonic's web bundle, the tests require internet access,
     # which is not available in the current environment.
   '';
@@ -84,4 +85,9 @@ lib.mapAttrs networkRequiringTestPkg {
     export HOME=$PWD
     tectonic -X compile ./test.tex
   '';
+
+  workspace = ''
+    tectonic -X new
+    cat Tectonic.toml | grep "${tectonic.bundleUrl}"
+  '';
 }