Show more information when test_installation fails

Previously, it attempted to show stderr unless empty, first falling
back to stdout unless empty, then falling back to the prewritten
summary identifying the specific assertion.

This now has the `test_installation` assertions capture stderr as
well as stdout, handle standard streams as text rather than binary,
and show more information when failing, always distinguishing where
the information came from: the summary, then labeled captured
stdout (empty or not), then labeled captured stderr (empty or not).

That applies to all but the last assertion, which does not try to
show information differently when it fails, but is simplified to do
the right thing now that `subprocess.run` is using text streams.
(This subtly changes its semantics, but overall it should be as
effective as before at finding the `sys.path` woe it anticipates.)

Author: EliahKagan

test/test_installation.py

@@ -15,31 +15,19 @@ def test_installation(self, rw_dir):
         venv, run = self._set_up_venv(rw_dir)
 
         result = run([venv.pip, "install", "."])
-        self.assertEqual(
-            0,
-            result.returncode,
-            msg=result.stderr or result.stdout or "Can't install project",
-        )
+        self._check_result(result, "Can't install project")
 
         result = run([venv.python, "-c", "import git"])
-        self.assertEqual(
-            0,
-            result.returncode,
-            msg=result.stderr or result.stdout or "Self-test failed",
-        )
+        self._check_result(result, "Self-test failed")
 
         result = run([venv.python, "-c", "import gitdb; import smmap"])
-        self.assertEqual(
-            0,
-            result.returncode,
-            msg=result.stderr or result.stdout or "Dependencies not installed",
-        )
+        self._check_result(result, "Dependencies not installed")
 
         # Even IF gitdb or any other dependency is supplied during development by
         # inserting its location into PYTHONPATH or otherwise patched into sys.path,
         # make sure it is not wrongly inserted as the *first* entry.
         result = run([venv.python, "-c", "import sys; import git; print(sys.path)"])
-        syspath = result.stdout.decode("utf-8").splitlines()[0]
+        syspath = result.stdout.splitlines()[0]
         syspath = ast.literal_eval(syspath)
         self.assertEqual(
             "",
@@ -63,8 +51,17 @@ def _set_up_venv(rw_dir):
         run = functools.partial(
             subprocess.run,
             stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            universal_newlines=True,
             cwd=venv.sources,
             env={**os.environ, "PYTHONWARNINGS": "error"},
         )
 
         return venv, run
+
+    def _check_result(self, result, failure_summary):
+        self.assertEqual(
+            0,
+            result.returncode,
+            f"{failure_summary}\nstdout:\n{result.stdout}\nstderr:\n{result.stderr}",
+        )