[test] Fix TestSourceManager when the source file is readonly.

This test copies main.c to main-copy.c and modifies main-copy.c while debugging, but main.c may have come from a readonly location, which means writing to main-copy.c will fail because permissions are preserved. Run the equivalent of "chmod u+w" before attempting to modify it.

This effect can be seen by attempting to run this test after running `chmod u-w lldb/test/API/source-manager/main.c`