Only add `darwin_log_cmd` lit shell test feature when the log can be queried.

Summary:
Follow up fix to 445b810f. 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.

rdar://problem/55986279

Reviewers: kubamracek, yln, dcoughlin

Subscribers: Charusso, #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D76899