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

[nomis]

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.

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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).

[nomis]

I expect this isn't trivial to do for all the environment variables because the test runner doesn't have them because it's a separate process, but it does know the PIO environment name and the program name which would be very useful for any wrapper script.