Tests: Make assertCapturedSuccess check stderr is empty again (#1087)

`assertCapturedSuccess` used to check that `stderr` was empty, until:
https://github.com/heroku/heroku-buildpack-python/commit/797652a75d69a1fece96a26bf4514fe9e9e1c020#diff-65c067a6f0a3aef292fb54ec21a1fe8cR98

Adding back the assertion exposes some new bugs, which I'll fix in
later PRs. In the meantime affected tests have been adjusted to use
a new `assertCapturedSuccessWithStdErr`.

The test harness output for the failing case has also been improved,
to ease debugging.

Closes @W-7918492@.

[skip changelog]

test/utils

@@ -85,12 +85,20 @@ assertNotCaptured()
 assertCapturedSuccess()
 {
   assertEquals "Captured exit code -" "0" "${RETURN}"
-  # assertEquals "STD_ERR -" "" "$(cat ${STD_ERR})"
+  assertEquals "STD_ERR -" "" "$(cat ${STD_ERR})"
 
-  if [ $RETURN -ne 0 -a -z "$(cat ${STD_ERR})" ]; then
-    # Failing exit code but stderr was empty. Display stdout to help debugging.
-    cat $STD_OUT
-    echo
+  if [ $RETURN -ne 0 -o -n "$(cat ${STD_ERR})" ]; then
+    debug
+  fi
+}
+
+assertCapturedSuccessWithStdErr()
+{
+  assertEquals "Captured exit code -" "0" "${RETURN}"
+  assertNotEquals "STD_ERR -" "" "$(cat ${STD_ERR})"
+
+  if [ ${RETURN} -ne 0 ]; then
+    debug
   fi
 }
 
@@ -151,9 +159,14 @@ _assertContains()
 
 debug()
 {
-  cat $STD_OUT
-  echo '^^^^^^'
-  cat $STD_ERR
+  echo
+  echo '### STD_OUT ###'
+  cat "${STD_OUT}"
+  echo
+  echo '### STD_ERR ###'
+  cat "${STD_ERR}"
+  echo
+  echo
 }
 
 assertContains()