docker: Bump default LLVM used for tests to 11

docker: Bump default LLVM used for tests to 11

This is another prep commit for running docker tests on both 18.04 and
20.04 versions of ubuntu. 20.04 doesn't have some LLVM 8 libs that the
docker build expects, so bump to 11.

Bumping to 11 causes test_disassembler to fail because it expects a map
w/ a certain format with FD 3, and the LLVM bump causes the map to be
FD 4. The purpose of the test is to ensure that the format of the
diassembled map is correct, not that it has a specific FD, so make some
small changes to the test so that it doesn't care what the FD number is.