Fix paths based on the assumption that the CMake build tree is in
${CMAKE_SOURCE_DIR}/build.

See-also: https://github.com/GothenburgBitFactory/taskwarrior/pull/3998
From: Lucio Sauer <watermanpaint@posteo.net>
--- a/test/bash_completion.test.py
+++ b/test/bash_completion.test.py
@@ -34,9 +34,9 @@ from contextlib import contextmanager
 sys.path.append(os.path.dirname(os.path.abspath(__file__)))
 
 from basetest import Task, TestCase
-from basetest.utils import BIN_PREFIX
+from basetest.utils import SOURCE_DIR
 
-TASKSH = os.path.abspath(os.path.join(BIN_PREFIX, "..", "..", "scripts/bash/task.sh"))
+TASKSH = os.path.abspath(os.path.join(SOURCE_DIR, "scripts/bash/task.sh"))
 
 
 @contextmanager
--- a/test/tw-1379.test.py
+++ b/test/tw-1379.test.py
@@ -32,8 +32,7 @@ import unittest
 sys.path.append(os.path.dirname(os.path.abspath(__file__)))
 
 from basetest import Task, TestCase
-
-REPO_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+from basetest.utils import SOURCE_DIR
 
 
 class TestBug1379(TestCase):
@@ -41,7 +40,7 @@ class TestBug1379(TestCase):
         self.t = Task()
         # Themes are a special case that cannot be set via "task config"
         with open(self.t.taskrc, "a") as fh:
-            fh.write("include " + REPO_DIR + "/../doc/rc/no-color.theme\n")
+            fh.write("include " + SOURCE_DIR + "/doc/rc/no-color.theme\n")
 
         self.t.config("color.alternate", "")
         self.t.config("_forcecolor", "1")