testing: fix layer tests when run with tox

This commit changes the way how the `versions.cfg` file, which contains
the currently used CrateDB version for testing, is located at the
project root.

Author: chaudum

src/crate/testing/doctests/layer.txt

@@ -230,7 +230,7 @@ tarball::
 
     >>> import configparser
     >>> config = configparser.ConfigParser()
-    >>> config.read(os.path.join(os.getcwd(), "..", "..", "versions.cfg"))
+    >>> config.read(project_root("versions.cfg"))
     ['.../versions.cfg']
 
     >>> version = config["versions"]["crate_server"]

src/crate/testing/tests.py

@@ -30,14 +30,27 @@
 
 
 def docs_path(*parts):
-    return os.path.join(os.path.dirname(os.path.dirname(__file__)), *parts)
+    return os.path.abspath(
+        os.path.join(
+            os.path.dirname(os.path.dirname(__file__)), *parts
+        )
+    )
+
+
+def project_root(*parts):
+    return os.path.abspath(
+        os.path.join(docs_path("..", ".."), *parts)
+    )
 
 
 def crate_path(*parts):
-    return os.path.abspath(docs_path('..', '..', 'parts', 'crate', *parts))
+    return os.path.abspath(
+        project_root("parts", "crate", *parts)
+    )
 
 
 def setUp(test):
+    test.globs['project_root'] = project_root
     test.globs['crate_path'] = crate_path
     test.globs['tempfile'] = tempfile
     test.globs['os'] = os

versions.cfg

@@ -16,7 +16,7 @@ createcoverage = 1.2
 docutils = 0.12
 hexagonit.recipe.download = 1.7
 py = 1.4.26
-tox = 1.8.1
+tox = 3.7.0
 twine = 1.8.1
 virtualenv = 15.1.0
 wheel = 0.24.0