Node.js: ServerResponse.flushHeaders() implemented.

This closes #1006 issue on GitHub.

Reviewed-by: Andrew Clayton <a.clayton@nginx.com>

Node.js: response body chunk can now be a Uint8Array.

Starting from Node.js 15.0.0 the chunk parameter of the response.write()
can be a Uint8Array.

This closes #870 issue on GitHub.

Tests: get rid of classes in test files.

Class usage came from the unittest framework and it was always redundant
after migration to the pytest. This commit removes classes from files
containing te

Tests: get rid of classes in test files.

Class usage came from the unittest framework and it was always redundant
after migration to the pytest. This commit removes classes from files
containing tests to make them more readable and understandable.

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Tests: prerequisites checking reworked.

Prerequisites check moved to the module level to simplify class structure.
Discovery and prerequisites checks functions moved to the separate files.
Introduce

Tests: prerequisites checking reworked.

Prerequisites check moved to the module level to simplify class structure.
Discovery and prerequisites checks functions moved to the separate files.
Introduced "require" fixture to provide per-test requirements check.

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Tests: more fixtures.

Common methods from applications/proto.py converted to the fixtures.
sysctl check moved to the specific file where it is using.
Some options moved to the constructor to have ea

Tests: more fixtures.

Common methods from applications/proto.py converted to the fixtures.
sysctl check moved to the specific file where it is using.
Some options moved to the constructor to have early access.

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Tests: switched to using f-strings.

Previously, it was necessary to support older versions of Python for
compatibility. F-strings were released in Python 3.6. Python 3.5 was
marked as unsupported

Tests: switched to using f-strings.

Previously, it was necessary to support older versions of Python for
compatibility. F-strings were released in Python 3.6. Python 3.5 was
marked as unsupported by the end of 2020, so now it's possible to start
using f-strings safely for better readability and performance.

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Tests: minor fixes.

Tests: style.

Tests: style.

Node.js: renamed "require_shim" to "loader".

Node.js: a shim for overriding "http" and "websocket" modules.

Also added stubs for Server.address()
This was done to prevent crashes in some popular frameworks like express

Supports both CommonJS

Node.js: a shim for overriding "http" and "websocket" modules.

Also added stubs for Server.address()
This was done to prevent crashes in some popular frameworks like express

Supports both CommonJS and the new ES Modules system syntax e.g:

app.js:
const http = require('http')

app.mjs:
import http from "http"

Usage on Node 14.16.x and higher:
{
"type": "external",
"processes": {"spare": 0},
"working_directory": '/project',
"executable": "/usr/bin/env",
"arguments": [
"node",
"--loader",
"unit-http/require_shim.mjs"
"--require",
"unit-http/require_shim",
"app.js"
]
}

Usage on Node 14.15.x and lower:
{
"type": "external",
"processes": {"spare": 0},
"working_directory": '/project',
"executable": "/usr/bin/env",
"arguments": [
"node",
"--require",
"unit-http/require_shim",
"app.js"
]
}

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Tests: style.

Tests: style.

Node.js: ServerRequest and ServerResponse compliance to Stream API.

ServerRequest now inherit stream Readable object. ServerResponse
provides 'writable' property.

Thanks to Wu Jian Ping (@wujjpp).

Node.js: ServerRequest and ServerResponse compliance to Stream API.

ServerRequest now inherit stream Readable object. ServerResponse
provides 'writable' property.

Thanks to Wu Jian Ping (@wujjpp).

This closes #274, closes #317 issues and closes #502 PR on GitHub.

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Tests: utils module introduced.

Tests: fixed unit.log print.

Tests: minor fixes.

Tests: migrated to the pytest.

Tests: style.

Tests: introduced module version specification in prerequisites.

Tests: minor fixes and style.

Tests: parsing of "Transfer-Encoding: chunked" responses.

Tests: refactored prerequisites model.

Tests: adjusted skipping tests.

Tests: speed up tests.