Tests/CI: First portion of ASan/leak fixes

Forked from !932 (closed), this is hopefully the low-hanging fruit which:

• adds a LeakSanitizer suppression for the older version of fontconfig shipped in Debian when forking for Xwayland
• adds a workaround to disable ASan's fast-unwind tracer when suppressions are required, since frame pointers are omitted so the suppressions are not effective with the fast tracer
• avoids llvmpipe ever being unloaded from our address space, in order for ASan to not complain about LLVM's type-system initialisation
• deletes ZUC's support for forking every single subtest as a new process, since this takes ~3.6s on aarch64 when ASan is active, and is of no use to us anyway
• switches the config-parser test to use a memstream for its config files, rather than constantly creating and destroying small files on-disk, which can be a surprisingly bad I/O bottleneck
• fixes a leak in all non-ZUC test runs, and further fixes another leak when tests are being skipped
• fixes an issue in the Xwayland window manager where we would blindly assume a client had sent the WM_NORMAL_HINTS structure as full size, even though it can be smaller
• fixes a leak of forked-process data in Xwayland support