python: Add runnable unittest tasks (#12451)

Add runnable tasks for Python, starting with `unittest` from the
standard library. Both `TestCase`s (classes meant to be a unit of
testing) and individual test functions in a `TestCase` will have
runnable icons. For completeness, I also included a task that will run
`unittest` on the current file.

The implementation follows the `unittest` CLI. The unittest module can
be used from the command line to run tests from modules, classes or even
individual test methods:

```
python -m unittest test_module.TestClass
python -m unittest test_module.TestClass.test_method
```

```python
import unittest

class TestStringMethods(unittest.TestCase):

    def test_upper(self):
        self.assertEqual('foo'.upper(), 'FOO')

    def test_isupper(self):
        self.assertTrue('FOO'.isupper())
        self.assertFalse('Foo'.isupper())

    def test_split(self):
        s = 'hello world'
        self.assertEqual(s.split(), ['hello', 'world'])
        # check that s.split fails when the separator is not a string
        with self.assertRaises(TypeError):
            s.split(2)

if __name__ == '__main__':
    unittest.main()
```

From the snippet provided by `unittest` docs, a user may want to run
test_split independently of the other test functions in the test case.
Hence, I decided to make each test function runnable despite `TestCase`s
being the unit of testing.

## Example of running a `TestCase`
<img width="600" alt="image"
src="https://github.com/zed-industries/zed/assets/16619392/7be38b71-9d51-4b44-9840-f819502d600a">

## Example of running a test function in a `TestCase`
<img width="600" alt="image"
src="https://github.com/zed-industries/zed/assets/16619392/f0b6274c-4fa7-424e-a0f5-1dc723842046">

`unittest` will also run the `setUp` and `tearDown` fixtures.

Eventually, I want to add the more commonly used `pytest` runnables
(perhaps as an extension instead).

Release Notes:

- Added runnable tasks for Python `unittest`.
([#12080](https://github.com/zed-industries/zed/issues/12080)).

crates/languages/src/lib.rs

@@ -3,6 +3,7 @@ use gpui::{AppContext, UpdateGlobal};
 use json::json_task_context;
 pub use language::*;
 use node_runtime::NodeRuntime;
+use python::PythonContextProvider;
 use rust_embed::RustEmbed;
 use settings::SettingsStore;
 use smol::stream::StreamExt;
@@ -10,10 +11,7 @@ use std::{str, sync::Arc};
 use typescript::typescript_task_context;
 use util::{asset_str, ResultExt};
 
-use crate::{
-    bash::bash_task_context, go::GoContextProvider, python::python_task_context,
-    rust::RustContextProvider,
-};
+use crate::{bash::bash_task_context, go::GoContextProvider, rust::RustContextProvider};
 
 mod bash;
 mod c;
@@ -130,7 +128,7 @@ pub fn init(
         vec![Arc::new(python::PythonLspAdapter::new(
             node_runtime.clone(),
         ))],
-        python_task_context()
+        PythonContextProvider
     );
     language!(
         "rust",