Fix test wrt. opcache.error_log

Since the test expects log messages to be written to stderr, we have to
make sure that opcache.error_log is set correctly when running the
test.

Author: cmb69

ext/opcache/tests/zzz_basic_logging.phpt

@@ -7,6 +7,7 @@ outputs the correct logging at the highest log_verbosity_level
 opcache.enable=1
 opcache.enable_cli=1
 opcache.file_cache_only=0
+opcache.error_log=
 opcache.log_verbosity_level=4
 opcache.huge_code_pages=0
 --SKIPIF--