[dfsan] Add test case for sscanf (#94700)

This test case shows a limitation of DFSan's sscanf implementation
(introduced in https://reviews.llvm.org/D153775): it simply ignores
ordinary characters in the format string, instead of actually comparing
them against the input. This may change the semantics of instrumented
programs.

Importantly, this also means that DFSan's release_shadow_space.c test,
which relies on sscanf to scrape the RSS from /proc/maps output, will
incorrectly match lines that don't contain RSS information. As a result,
it adding together numbers from irrelevant output (e.g., base
addresses), resulting in test flakiness
(https://github.com/llvm/llvm-project/issues/91287).