1. Fix PICOLIB_AUTORUN env var naming.
2. Add an assertion that it works as expected.
3. Show output if listing tests fails.
3. Run inner tests with listen port :0 to avoid
   "address already in use" errors.

Tarantool specificities make it difficult to set up usual rust tests.
Picolib is coupled with tarantool symbols and compiled as a dylib, so
it can't be linked to a stand-alone binary.

This patch introduces another way of testing.

All tests (called inner) are already built into dylib and exposed in
the tarantool console:

```
tarantool> picolib.test
---
- test_version: 'function: 0x41f32400'
  test_traft_storage_RaftEntryRow: 'function: 0x41f32550'
...
```

The new integration test `inner.rs` iterates over them and runs each
one in a separate tarantool instance.

See also: commit 6610e5cb

The clap module we use for parsing command-line arguments allows
specifying `default_value`. Unfortunately, it makes impossible to tell
if an argument was actually passed. So it always overrides env var with
a default value, which is undesirable.

The control flow of picodata looks as following:

Picodata binary -> exec tarantool -l picodata -> libpicodata.so -> ...

Since the libpicodata is a shared lib loaded by tarantool, we can't pass
it arguments as usual. At first, tarantool parses them by itself and
rejects uknown arguments. Secondly, it spoils argv array.

To overcome these two issues, this patch introduces argparse on behalf
of the picodata binary, and puts parsed values into the environment
variables.

P.S. Thihs is how command line args look from within libpicodata:

```console
$ tarantool -l picodata /dev/null --opt1 foo --opt2 bar
Hello from rust lib
arg[0] tarantool
arg[1] /dev/null
arg[2] --opt1
arg[3] foo
arg[4] --opt2
arg[5] bar
arg[6] --opt2
arg[7] bar
```

This patch introduces the new cargo package - picodata.

The package consists of two parts:
- The `picodata` binary does some env magic and executes
  `tarantool -l picodata`.
- The `libpicodata` dylib is the starting point for further
  development.