[scudo] Delay allocations in the RSS check test

Summary:
D57116 fails on the armv7 bots, which is I assume due to the timing of
the RSS check on the platform. While I don't have a platform to test
that change on, I assume this would do.

The test could be made more reliable by either delaying more the
allocations, or allocating more large-chunks, but both those options
have a somewhat non negligible impact (more memory used, longer test).

Hence me trying to keep the additional sleeping/allocating to a
minimum.

Reviewers: eugenis, yroux

Reviewed By: yroux

Subscribers: javed.absar, kristof.beyls, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D57241

llvm-svn: 352220