python3Packages.nmigen: fix build

Tried to debug failure & traced it to subprocess.Popen calling fork_exec()
with a bad path, but couldn't reproduce error from manual debugging
using ``nix-shell``.

Bad path meaning that most paths were of form /nix/store/.../bin/sby,
but one path was "sby" only. That's my only guess as to what would
be causing this failure.

pkgs/development/python-modules/nmigen/default.nix

@@ -8,6 +8,7 @@
 , jinja2
 
 # for tests
+, pytestCheckHook
 , yosys
 , symbiyosys
 , yices
@@ -18,6 +19,7 @@ buildPythonPackage rec {
   version = "unstable-2020-04-02";
   # python setup.py --version
   realVersion = "0.2.dev49+g${lib.substring 0 7 src.rev}";
+  disabled = pythonOlder "3.6";
 
   src = fetchFromGitHub {
     owner = "nmigen";
@@ -26,18 +28,23 @@ buildPythonPackage rec {
     sha256 = "sha256-3+mxHyg0a92/BfyePtKT5Hsk+ra+fQzTjCJ2Ech44/s=";
   };
 
-  disabled = pythonOlder "3.6";
-
   nativeBuildInputs = [ setuptools_scm ];
 
   propagatedBuildInputs = [ setuptools pyvcd jinja2 ];
 
-  checkInputs = [ yosys symbiyosys yices ];
+  checkInputs = [ pytestCheckHook yosys symbiyosys yices ];
 
   preBuild = ''
     export SETUPTOOLS_SCM_PRETEND_VERSION="${realVersion}"
   '';
 
+  # Fail b/c can't find sby (symbiyosys) executable, which should be on path.
+  disabledTests = [
+    "test_distance"
+    "test_reversible"
+    "FIFOFormalCase"
+  ];
+
   meta = with lib; {
     description = "A refreshed Python toolbox for building complex digital hardware";
     homepage = "https://nmigen.info/nmigen";