Fix test issue-1831.json on LMDB

When LMDB is enabled, ModSecurity stores its persistent variables in
"./modsec-shared-collections" file. Since this file wasn't cleared between
tests, tests behaved differently on "in-memory per-process" and LMDB backend.

This test never worked in LMDB configuration. It hasn't been discovered
until now because Travis CI didn't test LMDB configuration when test was
introduced.

Author: WGH-

test/regression/regression.cc

@@ -15,6 +15,8 @@
 
 #include <string.h>
 
+#include <unistd.h>
+
 #include <ctime>
 #include <iostream>
 #include <string>
@@ -136,6 +138,12 @@ void perform_unit_test(ModSecurityTest<RegressionTest> *test,
             continue;
         }
 
+#ifdef WITH_LMDB
+        // some tests (e.g. issue-1831.json)  don't like it when data persists between runs
+        unlink("./modsec-shared-collections");
+        unlink("./modsec-shared-collections-lock");
+#endif
+
         modsec = new modsecurity::ModSecurity();
         modsec->setConnectorInformation("ModSecurity-regression v0.0.1-alpha" \
             " (ModSecurity regression test utility)");