Expose install_kernel for tests

[jtpio]

References

Following up on #6776 and #6770 to make the test framework reusable outside of JupyterLab.

Code changes

ProcessTestApp now exposes the install_kernel(kernel_name, kernel_spec) method.

Developer-facing changes

This change exposes the install_kernel method to be able to install extra kernel specs other than the default ones.

This is useful for applications and third-party extensions reusing the JestApp from jupyterlab.tests.test_app to run tests that require a specific kernel.

Example

jest_app = JestApp.instance()
jest_app.jest_dir = HERE
jest_app.install_kernel(
    kernel_name="xpython",
    kernel_spec={
        'argv': [
            'xpython',
            '-f', '{connection_file}'
        ],
        'display_name': "xpython",
        'language': 'python'
    }
)
jest_app.initialize()
jest_app.start()

User-facing changes

None

Backwards-incompatible changes

None

[jupyterlab-dev-mode]

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[blink1073]

LGTM, thanks!