Ensure that ReplSetTests in tenant migration tests have the intended node options

There are a number of tenant migration tests that use Object.assign() to create nodeOptions from the global migrationX509Options, for example this one. Object.assign() mutates the first argument and returns it so to make each test case use the intended nodeOptions, Object.assign(migrationX509Options...., ...) should be changed to Object.assign({}, migrationX509Options..., ...) instead.