Re-implement enum documentation test

[adam-enko]

Re-implement the enum documentation test to download and use the actual Kotlin stdlib enum source code for the synthetic enum functions.

This avoids hardcoding a URL, and helps ensure the tests are properly cacheable (so they don't dynamically download files at runtime).

• Add a utility, downloadKotlinStdlibJvmSources(), to download the Kotlin stdlib source code.
• Move StdLibDocumentationIntegrationTest.kt to plugin-base since it's not really an integration test, and otherwise it's hard to access the actual enum template files.
• Update the enum template tests to test all templates.

[whyoleg]

Thanks!
The approach looks good to me, the only problem I see is with version of stdlib which is used for comparing.
We used master version, but now we will use some specific version, so we will lag behind a bit. Not sure it will really matter.

[IgnatBeresnev]

Move StdLibDocumentationIntegrationTest.kt to plugin-base since it's not really an integration test, and otherwise it's hard to access the actual enum template files.

For context: I added it as an integration test became it made a network request that could hang, return an error or produce unexpected results. That's somewhat expected from integration (e2e) tests, but I certainly wouldn't expect unit tests to fail if GitHub is down

[dkrasnoff]

Good job! 👍