Provide standard `PIO*` and `PLATFORMIO_*` environment variables when running test command

What kind of issue is this?

• Feature Request.
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  already exists! Don’t send pull requests to implement new features without first getting our
  support. Sometimes we leave features out on purpose to keep the project small.

Configuration

PlatformIO Version (platformio --version): 6.1.14

Description of problem

Making a custom wrapper around test_testing_command is inconvenient because PlatformIO core has access to information that variables do not, like the current environment name (not just the section name) and the program name (which isn't program on Windows).

If the test command was executed with the PIOENV environment variable then a script would know what the current environment is. An additional variable could contain the program name (although any script is unlikely to work on both Windows and non-Windows environments).

Start with the test_testing_command documentation and the test-command entry point, using PlatformIO 6.1.14 as the reported baseline. Trace how the current environment and program name are exposed, then verify that PIOENV and the requested PIO*/PLATFORMIO_* variables are available to wrapper commands on supported platforms.