[lldb] Fix test_exec_root of API tests

lit has grown a feature where it stores the runtimes of all tests.
Normally, these times should be stored in the build directory, but
because our API tests have set test_exec_root to point to the source
tree, it has ended up polluting our checkout and led to the
.lit_test_times.txt being committed to the repository.

Delete this file, and adjust the exec root of API tests. I've also
needed to adjust the root of Shell tests, in order to avoid the two
overlapping.