Add end-to-end continuous integration tests for Homebrew distribution

[Lukas-Stuehrk]

This changes the end-to-end tests so it's possible to use the tests with the swift-doc version installed via homebrew.

It also adds a new GitHub actions workflow to run the end-to-tests with the homebrew version of swift-doc, both from bottle and from source.

This implements #202.

Unfortunately, the integration tests will fail. I can reproduce this also on my machine when I use the swift-doc version from homebrew:

lukas@grumpycat ~/src/swift-doc $ which swift-doc                                                                           
/Users/lukas/homebrew/bin/swift-doc
lukas@grumpycat ~/src/swift-doc $ swift-doc generate --module-name SwiftDoc --format commonmark --output /tmp/docs .
[1]    47753 illegal hardware instruction  swift-doc generate --module-name SwiftDoc --format commonmark --output  .

The crashing line is

swift-doc/Sources/SwiftDoc/SourceFile.swift

Line 141 in f441648

assert(node.parent?.parent?.is(IfConfigDeclSyntax.self) == true)

[mattt]

Unfortunately, the integration tests will fail. I can reproduce this also on my machine when I use the swift-doc version from homebrew:

lukas@grumpycat ~/src/swift-doc $ which swift-doc                                                                           
/Users/lukas/homebrew/bin/swift-doc
lukas@grumpycat ~/src/swift-doc $ swift-doc generate --module-name SwiftDoc --format commonmark --output /tmp/docs .
[1]    47753 illegal hardware instruction  swift-doc generate --module-name SwiftDoc --format commonmark --output  .

The crashing line is

swift-doc/Sources/SwiftDoc/SourceFile.swift

Line 141 in f441648

assert(node.parent?.parent?.is(IfConfigDeclSyntax.self) == true)

Huh, that's surprising. I thought Homebrew built and installed in release mode, so assertions would be disabled. Let me double-check what we're doing in our formula. At the very least, I can remove that assertion — that's something we could and probably should be validating in a unit test instead.

[mattt]

Unfortunately, the integration tests will fail. I can reproduce this also on my machine when I use the swift-doc version from homebrew...

Oh wait, I recognize that crash! That's what happens when you try to run swift-doc on SwiftSyntax (or one of the other dependencies). Fortunately, this is fixed by #229. We can hold off on merging this until that gets into an official release.