Only skip null bytes before setting the environment variable

As discussed, node ids have already been "ascii" sanitized by the
parametrization process

_pytest/runner.py

@@ -7,7 +7,7 @@ import sys
 from time import time
 
 import py
-from _pytest.compat import _ascii_escaped, _PY2
+from _pytest.compat import _PY2
 from _pytest._code.code import TerminalRepr, ExceptionInfo
 from _pytest.outcomes import skip, Skipped, TEST_OUTCOME
 
@@ -135,7 +135,7 @@ def _update_current_test_var(item, when):
     """
     var_name = 'PYTEST_CURRENT_TEST'
     if when:
-        value = _ascii_escaped('{0} ({1})'.format(item.nodeid, when))
+        value = '{0} ({1})'.format(item.nodeid, when)
         if _PY2:
             # python 2 doesn't like null bytes on environment variables (see #2644)
             value = value.replace('\x00', '(null)')