Only add darwin_log_cmd lit shell test feature when the log can be queried.
Follow up fix to 445b810fbd4. The log show command only works for
privileged users so run a quick test of the command during lit config to
see if the command works and only add the darwin_log_cmd feature if
this is the case.
Unfortunately this means the asan/TestCases/Darwin/duplicate_os_log_reports.cpp
test and any other tests in the future that use this feature won't run
for unprivileged users which is likely the case in CI.
Reviewers: kubamracek, yln, dcoughlin
Subscribers: Charusso, #sanitizers, llvm-commits
Differential Revision: https://reviews.llvm.org/D76899