[lldb] [lit] Introduce tests for writing x86 general-purpose registers

Introduce two initial tests for 'register write' command.  The tests
first clobber x86 general purpose registers, then call int3 to let lldb
write to them, then print the new values.  FileCheck takes care of
verifying whether correct values were written.

Differential Revision: https://reviews.llvm.org/D61221

llvm-svn: 359441