gnu: behave: Fix tests.

* gnu/packages/check.scm (behave): Add missing directory to PYTHONPATH before
tests.

gnu/packages/check.scm

@@ -1962,7 +1962,15 @@ (define-public behave
        ("python-parse" ,python-parse)
        ("python-parse-type" ,python-parse-type)))
     (arguments
-     '(#:test-target "behave_test"))
+     '(#:test-target "behave_test"
+       #:phases
+       (modify-phases %standard-phases
+         (add-before 'check 'fix-library-loading
+           (lambda _
+             ;; Otherwise, tests fail with no module named 'path'
+             (setenv "PYTHONPATH" (string-append (getenv "PYTHONPATH") ":"
+                                                 (getcwd) "/tasks/_vendor"))
+             #t)))))
     (home-page "https://github.com/behave/behave")
     (synopsis "Python behavior-driven development")
     (description