Add a document describing our environment variable usage. (#1300)

Adds a document describing our environment variable usage for reference
by developers working on Swift Testing.

This document is not an API contract: the set of environment variables
Swift Testing uses may change at any time.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

---------

Co-authored-by: Joseph Heck <[email protected]>

Author: grynspan

Documentation/EnvironmentVariables.md

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+<!--
+This source file is part of the Swift.org open source project
+
+Copyright (c) 2023–2025 Apple Inc. and the Swift project authors
+Licensed under Apache License v2.0 with Runtime Library Exception
+
+See https://swift.org/LICENSE.txt for license information
+See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+-->
+
+# Environment variables in Swift Testing
+
+This document lists the environment variables that Swift Testing currently uses.
+This list is meant for use by developers working on Swift Testing.
+
+Those environment variables marked with `*` are defined by components outside
+Swift Testing. In general, environment variables that Swift Testing defines have
+names prefixed with `SWT_`.
+
+> [!WARNING]
+> This document is not an API contract. The set of environment variables Swift
+> Testing uses may change at any time.
+
+## Console output
+
+| Variable Name | Value Type | Notes |
+|-|:-:|-|
+| `COLORTERM`\* | `String` | Used to determine if the current terminal supports 24-bit color. Common across UNIX-like platforms. |
+| `NO_COLOR`[\*](https://no-color.org) | `Any?` | If set to any value, disables color output regardless of terminal capabilities. |
+| `SWT_ENABLE_EXPERIMENTAL_CONSOLE_OUTPUT` | `Bool` | Used to enable or disable experimental console output. |
+| `SWT_SF_SYMBOLS_ENABLED` | `Bool` | Used to explicitly enable or disable SF&nbsp;Symbols support on macOS. |
+| `TERM`[\*](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html) | `String` | Used to determine if the current terminal supports 4- or 8-bit color. Common across UNIX-like platforms. |
+
+## Error handling
+
+| Variable Name | Value Type | Notes |
+|-|:-:|-|
+| `SWT_FOUNDATION_ERROR_BACKTRACING_ENABLED` | `Bool` | Used to explicitly enable or disable error backtrace capturing when an instance of `NSError` or `CFError` is created on Apple platforms. |
+| `SWT_SWIFT_ERROR_BACKTRACING_ENABLED` | `Bool` | Used to explicitly enable or disable error backtrace capturing when a Swift error is thrown. |
+
+## Event streams
+
+| Variable Name | Value Type | Notes |
+|-|:-:|-|
+| `SWT_EXPERIMENTAL_EVENT_STREAM_FIELDS_ENABLED` | `Bool` | Used to explicitly enable or disable experimental fields in the JSON event stream. |
+| `SWT_PRETTY_PRINT_JSON` | `Bool` | Used to enable pretty-printed JSON output to the event stream (for debugging purposes). |
+
+## Exit tests
+
+| Variable Name | Value Type | Notes |
+|-|:-:|-|
+| `SWT_BACKCHANNEL` | `CInt`/`HANDLE` | A file descriptor (handle on Windows) to which the exit test's events are written. |
+| `SWT_CAPTURED_VALUES` | `CInt`/`HANDLE` | A file descriptor (handle on Windows) containing captured values passed to the exit test. |
+| `SWT_CLOSEFROM` | `CInt` | Used on OpenBSD to emulate `posix_spawn_file_actions_addclosefrom_np()`. |
+| `SWT_EXIT_TEST_ID` | `String` (JSON) | Specifies which exit test to run. |
+| `XCTestBundlePath`\* | `String` | Used on Apple platforms to determine if Xcode is hosting the test run. |
+
+## Miscellaneous
+
+| Variable Name | Value Type | Notes |
+|-|:-:|-|
+| `CFFIXED_USER_HOME`\* | `String` | Used on Apple platforms to determine the user's home directory. |
+| `HOME`[\*](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html) | `String` | Used to determine the user's home directory. |
+| `SIMULATOR_RUNTIME_BUILD_VERSION`\* | `String` | Used when running in the iOS (etc.) Simulator to determine the simulator's version. |
+| `SIMULATOR_RUNTIME_VERSION`\* | `String` | Used when running in the iOS (etc.) Simulator to determine the simulator's version. |
+| `SWT_USE_LEGACY_TEST_DISCOVERY` | `Bool` | Used to explicitly enable or disable legacy test discovery. |