[th/copy-last-report] runtest: copy the last test report to a well-known place

Running the tests writes the result HTML to /tmp. That's useful, however:

• the generated file contains the test name. So if you run two tests, you need to lookup the right filename. In practice, during manual debugging of the tests, you care about the last run. Add the result to a well-known place (in .tmp/).

• the test results are in /tmp, which is a locally mounted tempfs. When using the "nm-in-container.sh" script, the tests run inside a container and the NetworkManager-ci directory is bind mounted and shared with the outer host. As the test result is only on /tmp inside the container, you would have to first copy the file out of the container. Instead, copy the file inside the "NetworkManager-ci/.tmp/" directory which is conveniently accessibly to the user of such a setup.