d/tests/check-signatures: Fix subprocess cleanup

With Python 3.13, reopening the stdout file descriptor from dpkg -s linux-source causes cleanup to fail later. It's simpler to call subprocess.Popen with text=True so that we get text output directly.

Using the context manager style isn't strictly required to fix this bug, but it results in much clearer error messages when things go wrong.

Closes: #1092796