test: enable parallel run for long test suites

Also enabled it for luajit-tap.

Part of #3232

test/engine_long/suite.ini

@@ -6,4 +6,4 @@ long_run =  delete_replace_update.test.lua delete_insert.test.lua
 lua_libs = suite.lua
 use_unix_sockets = True
 config = engine.cfg
-is_parallel = False
+is_parallel = True

test/long_run-py/suite.ini

@@ -7,4 +7,4 @@ valgrind_disabled =
 release_disabled =
 lua_libs = suite.lua
 use_unix_sockets = True
-is_parallel = False
+is_parallel = True

test/luajit-tap/suite.ini

 [default]
 core = app
 description = Luajit tests
-is_parallel = False
+is_parallel = True