tests: Don't leak WCFS log files

By default every WCFS run creates several files in /tmp/wcfs.*.log.* and
without explicit cleanup those files are left hanging on testnodes. Over
last ~6 months we accumulated ~ 300K such files.

Don't allow those files to be leaked by instructing WCFS to log to
stderr during test run. This should be also useful to see details in the
test output.

conftest.py

@@ -24,7 +24,7 @@ import pytest
 import transaction
 from golang import func, defer
 from functools import partial
-import gc
+import os, gc
 
 # reset transaction synchronizers before every test run.
 #
@@ -46,9 +46,24 @@ def transaction_reset():
     # nothing to run after test
 
 
+# prepend_env prepends prefix + ' ' to environment variable var.
+def prepend_env(var, prefix):
+    v = os.environ.get(var, '')
+    if v != '':
+        v = ' ' + v
+    v = prefix + v
+    os.environ[var] = v
+
+
 # enable log_cli on no-capture
 # (output during a test is a mixture of print and log)
 def pytest_configure(config):
+    # put WCFS log into stderr instead of to many files in /tmp/wcfs.*.log
+    # this way we don't leak those files and include relevant information in test output
+    #
+    # TODO put WCFS logs into dedicated dir without -v?
+    prepend_env('WENDELIN_CORE_WCFS_OPTIONS', '-logtostderr')
+
     if config.option.capture == "no":
         config.inicfg['log_cli'] = "true"
         assert config.getini("log_cli") is True