Run unit tests within Docker container on GitHub Actions

[swift-kim]

Goals:

• [x] Build unit tests for Tizen arm/arm64 architecture. - https://github.com/flutter-tizen/engine/pull/242
• [x] Run unit tests in a Tizen arm/arm64 Docker container on GitHub Actions. (Here's the proof of concept.)
  • https://github.com/flutter-tizen/engine/pull/241
  • https://github.com/flutter-tizen/engine/pull/244
• [x] Fix failing test cases. - https://github.com/flutter-tizen/engine/pull/240
• [x] Remove support for experimental Linux x64 builds. - https://github.com/flutter-tizen/engine/pull/248

Optional tasks:

• [x] Create a Format job. - https://github.com/flutter-tizen/engine/pull/243

cc @bbrto21 @bwikbs @WonyoungChoi

[bwikbs]

@swift-kim It seems like I'm asking too late, but I haven't been able to see this for a while due to personal circumstances. Q) Why did you remove x64? As I understand it, there is no use for it, is this correct? Any other reasons?

[swift-kim]

@bwikbs I feel sorry to delete some of the code that you worked on hard.😢 I wanted to ask your opinion before proceeding but I couldn't find you at your seat.

1. Yes, the main reason is that we can now run unit tests without the x64-specific code. Of course the code can be used for other purposes than testing, such as running normal Flutter apps on Linux desktop (EFL), but there has been no specific use case for it for a year.
2. The x64-specific code makes the entire code hard to maintain and less readable. Many of our new contributors have been asking why their PR build fails and how they should deal with build errors because it was hard to understand without sufficient background that they should write code for both Tizen and Linux. Writing and maintaining two sets of code is a burden even for those who are already familiar with the code.
3. We're writing a Flutter embedder for the Tizen platform but not (generic) Linux EFL. We've been using the Linux-specific code as a workaround for testing our embedder, but now we have no such demand, so there's no good reason to maintain it.
4. FYI, this activity is to be part of my Best Reviewer coursework.

[bwikbs]

@swift-kim First of all, I have no big affection for my work. (Basically I think they are sucks!) You do not have to worry about it at all. 😉 Thank you for your detailed explanation. I think that the parts mentioned are reasonable. A recent debugging environment has evolved and working on a Tizen device better than before. Still, I think it is fast and easy to use X64 shell for functional POCs. But It is not enough to keep it to be maintained.

BTW, I hope to have a good result of your BR work. Please let me know if you need something for it. :)