Adjust VMem limits in VMem tests following move to LCG 103

When moving to LCG 103, the VMem usage of these two tests jumped to 2.4 GB (strangely, the mergesmallfiles test is unaffected in the opt build but both are in the dbg builds). The cause is not understood (some change in the Root defaults?), and the tests on master are not affected. Decide to fix anyway, since we are no longer so sensitive to VMem (and in any case for the main Stripping use case we would use the corresponding stripping branches that do not use LCG 103.