[intel-pt] Improve the way the test determines whether to run

Summary:
@labath raised a concern on the way I was skipping this test. I think that was
fair and I found a better way.
Now I'm skipping if the CMAKE flag LLDB_BUILD_INTEL_PT is false.
I added an enabled_plugins entry in the dotest configuration, which gets
set by lit or lldb-dotest. The only available plugin right now is
'intel-pt', but I imagine it will be useful in the future for other
kinds of plugins that get determined at configuration time. I didn't
want to add a new argument option --enable-intel-pt or something or the
sort, as it wouldn't be useful for other cases.

Reviewers: labath, clayborg

Subscribers: lldb-commits, labath

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D77452