Update Test on Linux

[Kyle-Ye]

Bug/issue #, if applicable:

Summary

Even the Head() init compiler bug is fixed on Swift main. The Linux test is still failed on my local machine.

➜  swift-markdown git:(main) ✗ swift test
/root/swift-markdown/Tests/MarkdownTests/XCTestManifests.swift:30:27: error: cannot find 'testReplacing' in scope
        ("testReplacing", testReplacing),
...

➜  swift-markdown git:(main) ✗ swift test --generate-linuxmain
warning: '--generate-linuxmain' option is deprecated; tests are automatically discovered on all platforms
warning: can't discover tests on Linux; please use this option on macOS instead
/root/swift-markdown/Tests/MarkdownTests/XCTestManifests.swift:30:27: error: cannot find 'testReplacing' in scope
        ("testReplacing", testReplacing),
...

If we run swift test directly, we'll missing a lot of symbol since the tested API have change(Since as testReplacing). And according to XCTestManifests.swift we need to use swift test --generate-linuxmain to regenerate the test file.
But it still falls because swift test --generate-linuxmain is only valid on macOS now.

On macOS, after run the command the XCTestManifests.swift will be updated. But on Linux we can't discover tests on Linux before so the command on Linux is useless and did nothing.

Since tests are automatically discovered on all platforms now. We can simply remove the 2 files to solve the issue here.

Testing

Using the swift-5.5-release(without the Head() bug, however it should can be reproduced with the main branch swift) on Ubuntu 2004 on Sat Oct 23 13:14:59 UTC 2021

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Ran the ./bin/test script and it succeeded

[franklinsch]

@swift-ci test

[Kyle-Ye]

The test is success now. Can we merge it since all other PR's Linux test is blocked by this issue. @franklinsch