[lldb] Fix crash when completing register names after program exit

Previously the following would crash:
(lldb) run
Process 2594053 launched: '/tmp/test.o' (aarch64)
Process 2594053 exited with status = 0 (0x00000000)
(lldb) register read <tab>

As the completer assumed that the execution context would always
have a register context. After a program has finished, it does not.

Split out the generic parts of the test from the x86 specific tests,
and added "register info" to both.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D154413