tools: improve valgrind support

• Generate and use a list of suppressions that reduce noisiness for known (non-)issues.
• Use --zero-fill-buffers for tests, as they sometimes use Buffer.allocUnsafe() and valgrind reports that as usage of uninitialized memory.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines