1. This commit adds LLDB_TEST_PLATFORM_URL, LLDB_TEST_SYSROOT,
   LLDB_TEST_PLATFORM_WORKING_DIR, LLDB_SHELL_TESTS_DISABLE_REMOTE cmake flags
   to pass arguments for cross-compilation and remote running of both Shell&API
   tests.
2. To run Shell tests remotely, it adds 'platform select' and 'platform connect' commands to %lldb
   substitution.
3. 'remote-linux' feature added to lit to disable tests failing with remote execution.
4. A separate working directory is assigned to each test to avoid
   conflicts during parallel test execution.
5. Remote Shell testing is run only when LLDB_TEST_SYSROOT is set for
   building test sources. Recommended compiler for that is Clang.