appveyor: add _trial_temp/test.log as an "artifact"

Name it "trial_test_log.txt", so it can be read in a browser, not just
downloaded.

I'm hoping this will help track down why
test_magic_folder.RealTest.test_batched_process occasionally times out on
windows.

.appveyor.yml

@@ -37,8 +37,10 @@ after_test:
   # This builds the main tahoe wheel, and wheels for all dependencies.
   # Again, you only need build.cmd if you're building C extensions for
   # 64-bit Python 3.3/3.4. And you need to use %PYTHON% to get the correct
-  # interpreter
+  # interpreter. If _trial_temp still exists, the "pip wheel" fails on
+  # _trial_temp\local_dir (not sure why).
   - |
+    copy _trial_temp\test.log trial_test_log.txt
     rd /s /q _trial_temp
     %PYTHON%\python.exe setup.py bdist_wheel
     %PYTHON%\python.exe -m pip wheel -w dist .
@@ -48,6 +50,8 @@ artifacts:
   # "pip wheel -w dist ." puts all the dependency wheels there too
   # this gives us a zipfile with everything
   - path: 'dist\*'
+  - path: trial_test_log.txt
+    name: Trial test.log
 
 #on_success:
 #  You can use this step to upload your artifacts to a public website.