Clean up test suite under msan
Fixes a couple of test failures that triggered under msan with clang 11. This didn't reveal any real issues outside of tests.
Putting this in the CI is likely to be harder because:
- gcc doesn't support msan, even on the most recent version, so this will require clang to be installed
- By default wayland-scanner is also built with msan instrumentation, but it links to system libraries that haven't been. This causes it to crash with spurious errors.
I worked around 2 by building wayland-scanner, re-configuring the build, letting it fail, and copying the un-instrumented version over the instrumented version. A better solution will probably require restructuring the build system to support building wayland-scanner with different options to the library and tests.