`ut_*_subprocess` should signal readiness to `utf.py`

[aaaaalbert]

Unit tests may specify separate subprocess (i.e. server/service), setup, and shutdown scripts to be executed while/before/after test cases are executed. utf.py is responsible for the sequencing, i.e.

• Spawn ut_MODULE_subprocess.py and allow it 30 seconds to start
• Run ut_MODULE_setup.py
• Run the other ut_MODULE_* tests in alphabetical order
• Run ut_MODULE_shutdown.py
• Signal to ut_MODULE_subprocess.py to stop now

The first point is crucial, as no precautions are taken to verify that the subprocess has really started by the time the wait period elapsed. This bites us, e.g., in the nodemanager unit tests on OpenWrt.

utf.py should thus use active signalling for starting the subprocess, too.

[aaaaalbert]

This seems relatively easy to implement. Only the second and third part are actual additions.

• utf.py spawns ut_MODULE_subprocess.py
• utf.py blocks on the subprocess' stdout.readline()
• the subprocess prints something when it has started up, signalling utf.py to continue
• the subprocess blocks on its stdin.read() while utf.py runs through the test cases
• utf.py closes the subprocess' stdin
• the subprocess proceeds to finish.