Closes #70

Either execvp succeeds, in which case the process image is replaced, or
execvp throws an exception, in which case control flow exits the frame
anyway.

e.g. assert x == y -> self.assertEqual(x, y);
self.assertTrue(isinstance(x, y)) -> self.assertIsInstance(x, y)

These specific methods give more useful errors in the case of a test
failure.

This allows using features such as TestCase,assertIsInstance on Python
2.6 and earlier.

Although these are synonyms in Python 2.x, when using MyPy to typecheck
code use of file() causes spurious errors.

This commit also serves as one small step to Python 3.x compatibility,
since 3.x removes the file() builtin.

We use test.sh to consolidate test cases in one file so the README
would not be outdated easier rather than the hard-coded test commands
in a code block.

On my laptop (Ubuntu 17.10, Python 2.7.14 in a virtualenv),
`test_regular_mod` fails with

```
AssertionError: "\nimport sys\n\n\ndef say_hi():\n    print 'hi'\n" !=
'\x03\xf3\r\n\xbbW\xd5Yc\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00@\x00\x00\x00s\x19\x00\x00\x00d\x00\x00d\x01\x00l\x00\x00Z\x00\x00d\x02\x00\x84\x00\x00Z\x01\x00d\x01\x00S(\x03\x00\x00\x00i\xff\xff\xff\xffNc\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00C\x00\x00\x00s\t\x00\x00\x00d\x01\x00GHd\x00\x00S(\x02\x00\x00\x00Nt\x02\x00\x00\x00hi(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00sF\x00\x00\x00/home/alex/src/mitogen/tests/data/module_finder_testmod/regular_mod.pyt\x06\x00\x00\x00say_hi\x05\x00\x00\x00s\x02\x00\x00\x00\x00\x01(\x02\x00\x00\x00t\x03\x00\x00\x00sysR\x01\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00sF\x00\x00\x00/home/alex/src/mitogen/tests/data/module_finder_testmod/regular_mod.pyt\x08\x00\x00\x00<module>\x02\x00\x00\x00s\x02\x00\x00\x00\x0c\x03'
```

`__file__` contains the path of the compiled `.pyc`, not the `.py`
source file.

fixes #58

Since the above if block ends in a call to os.execv() this block will
only ever run when the if condition was false. Hence putting it in an
else clause is unnecessary.

Much uglier command line, but it works across major Python releases.

Ubuntu 17.04 provides Docker 1.12.6, which has API version 1.24.
`dev_requirements.txt` specifies the docker-py 2.5.1, which by default
requests API version 1.30.

Hence when the SSH unit tests try to run the container specified in
`DockerizedSshDaemon` an error occurs

```
APIError: 400 Client Error: Bad Request ("client is newer than server
(client API version: 1.30, server API version: 1.24)")
```

I presume that `u1704` is a relic of a bygone container/host.

On Ubuntu 17.10 something (probably Docker) appears to be accepting
connections, before sshd is fully ready. This results in a race
condition, and hence connection errors for the first few tests (2-3 on
my laptop).

testlib.wait_for_port() checks not only that the port can be connected
to, but also something resembling the sshd banner is sent.

Fixes #51