[lldb][test] Add --sysroot argument to dotest.py

This argument allows to set specific sysroot pass which will be used for
building LLDB API test programs.
It might come in handy for setting up cross-platform remote runs of API
tests on Windows host.

It can be useful for cross-compiling LLDB API tests. The argument can be
set using `LLDB_TEST_USER_ARGS` argument:
```
cmake ...
-DLLDB_TEST_USER_ARGS="...;--sysroot;C:\path\to\sysroot;..."
...
```