[dexter] Fix source-root-dir unittests on Windows

These tests were spuriously failing on Windows due to path separators getting
flipped from `/` to `\\`  in various parts of dexter:

test_add_breakpoint_with_source_root_dir
test_get_step_info
test_get_step_info_no_source_root_dir

Tested on Windows and Linux.

Patch written by @TWeaver.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D115338