Set error_log to an empty value if the test relies on that feature

Some tests fail if the error_log is overriden by the loaded ini
configuration. Explicitly set it to an empty value to prevent the
failures.
See php#10737 (comment)

Closes phpGH-10772.

Author: ndossche

Zend/tests/bug39542.phpt

@@ -1,5 +1,7 @@
 --TEST--
 Bug #39542 (Behaviour of require_once/include_once different to < 5.2.0)
+--INI--
+error_log=
 --FILE--
 <?php
 $oldcwd = getcwd();

Zend/tests/bug79919.phpt

@@ -1,5 +1,7 @@
 --TEST--
 Bug #79919 (Stack use-after-scope in define())
+--INI--
+error_log=
 --EXTENSIONS--
 simplexml
 --FILE--

ext/opcache/tests/jit/bug80861.phpt

@@ -1,6 +1,7 @@
 --TEST--
 Bug #80839: PHP problem with JIT
 --INI--
+error_log=
 opcache.enable=1
 opcache.enable_cli=1
 opcache.jit_buffer_size=1M