[lldb][test] Speed up ProcessAttach test (#201530)
ProcessAttach is our slowest test and runs for about 70s. We spend 60s
in the autocontinue test waiting for the target program to terminate.
The reason we wait for the program is that our autocontinue test is not
running its command in async mode, and we wait after the attach for the
next breakpoint or the program terminates.
This patch makes the attach and autocontinue run in async mode so we
don't wait for the program to finish. This reduces the test time from
70s to about 10s.
It also replaces the assertTrue call that was supposed to be an
assertEqual, which made the test succeed even though the inferior
process already terminated.
