[lldb] [test] Fix qRegisterInfo lldb-server tests to handle missing registers

Fix qRegisterInfo tests to handle Exx error response when querying
registers that are not supported on the platform in question.  This
is how FreeBSD and NetBSD platforms reporting missing registers right
now, and there certainly is value from verifying the remaining
registers.

This change fixes the test for FreeBSD but NetBSD has other regressions
that still need to be researched.

Differential Revision: https://reviews.llvm.org/D91922