Determine if we can avoid global lock upgrade in dbtests/repltests.cpp

Currently in Base(), we take the global lock in IX mode via dbtests::WriteContextForTests, then later in deleteAll(), we upgrade the global lock to X mode via Lock::GlobalWrite. Given that it's just the test construct, this probably isn't a real problem, but we are working to ban upgrading the global lock in SERVER-60621, and this test fails as a result.