[lldb][test] Clear pexpect found var before checking again

If you run cmake without pexpect installed it errors as expected.
However, if you just `pip install pexpect` and cmake again it still
doesn't find it because it cached the result of the search.

Unset the result before looking for pexpect. So that this works
as expected:
cmake ...
pip3 install pexpect
cmake ...

lldb/test/CMakeLists.txt

@@ -30,6 +30,7 @@ endif()
 # LLDB tree. However, we delay the deletion of it from the tree in case
 # users/buildbots don't have the package yet and need some time to install it.
 if (NOT LLDB_TEST_USE_VENDOR_PACKAGES)
+  unset(PY_pexpect_FOUND CACHE)
   lldb_find_python_module(pexpect)
   if (NOT PY_pexpect_FOUND)
     message(FATAL_ERROR