Pin ruff version and fix sporadic double unlink in tests

[lucic71]

ruff got suddenly updated and our python linting broke. Pin ruff to a fixed version so linting is stable.

[lucic71]

While fixing the ruff version I encountered another problem: C++/unsafe/refcount all work with the same file declared in tmp, for example const char *path = "/tmp/cpp2rust_lseek_test.tmp";. This can lead to double unlinks which make tests fail sporadically.

To fix this, I moved all files from /tmp into the temporary build directory of each test, the second argument is cwd:

-        self.cpp_result = RunResult(*lit.util.executeCommand(str(self.cpp_bin)))
+        self.cpp_result = RunResult(
+           *lit.util.executeCommand(str(self.cpp_bin), str(self.tmp_dir))
+        )