[lldb] [test/Register] Use initial state for write tests

Reset registers to their 'initial' state instead of a semi-random
pattern in write tests.  While the latter might have been helpful
while debugging failures (i.e. to distinguish unmodified registers
from mistakenly written zeroes), the former makes it possible to test
whether xstate_bv field is written correctly when using XSAVE.

With this change, the four relevant tests start failing on NetBSD
without D90105.

Differential Revision: https://reviews.llvm.org/D90114