Tests improvements

• generate-sysroots: Avoid using undefined string escape sequences

  These are not raw strings, so we need to escape a single backslash as a double backslash. This wasn't previously detected because we weren't running lint tools against tests/*.py, but a subsequent commit will fix that omission.

• generate-sysroots: Wrap long lines

  These weren't previously detected because we weren't running lint tools against tests/*.py, but a subsequent commit will fix that omission.

• tests: Move testutils up a level

  Previously we were only using it for pressure-vessel, but it's also useful to be able to write test scripts in Python for other things.

• tests: Generalize TAP test template

  GLib-based tests want a --tap argument, but other test scripts generally will not. Similarly, it's fairly common to want to use an interpreter or wrapper around a test.

• tests: Add some infrastructure for running shell/Python script tests

  This isn't really used yet, but will be used in a future commit. It's useful to be able to write high-level integration tests in a friendlier language.

• tests: Don't assert an exact version match during as-installed tests

  When running tests against an installed Steam Runtime, it's inconvenient to have to make the tests and the installed programs match exactly.

• tests: Look for python3.5 or python3 if build-time python3 is not found

  This makes the tests more amenable to being copied to a test system and run "as-installed", and in particular being able to run them inside the LD_LIBRARY_PATH Steam Runtime using an external (OS-provided) Python executable.

• many-builds: Give a hint about uploading tests to a test machine

  We can run a subset of the installed-tests against the Steam Runtime with commands like:

    .../steam-runtime/run.sh -- ./test-architecture

  Some of them currently fail when run like this because they can't find their helper executables, but it's a good start on being able to do more systematic testing of the LD_LIBRARY_PATH runtime.