[intel-pt] Implement a basic test case

* This is a reattempted commit due to a previous builtbot failure

- Now using a env var to determine whether to run the test, as
someone might have built liblldbIntelFeatures.so without intelPT
support, which would make this test fail.

Summary:
Depends on D76872.

There was no test for the Intel PT support on LLDB, so I'm creating one, which
will help making progress on solid grounds.

The test is skipped if the Intel PT plugin library is not built.

Reviewers: clayborg, labath, kusmour, aadsm

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D77107