Moved Xtend core performance tests in xtend.performance.tests project

[LorenzoBettini]

In xtend.core.tests there are two performance test PerformanceTest and XtendIncrementalBuilderPerformanceTest that are "a bit in the way" when running tests in xtend.core.tests: there's no easy way to exclude them and the launch configurations were meant for the old gradle age, so they don't help anymore.

This PR:

• move those tests in the xtend.performance.tests project; in that project there are already a few performance tests; those are for the UI, but the above mentioned tests are still related to building so it does not harm to put them in the performance.tests project; I also renamed the name in the POM without the "IDE"
• update the launch configurations in xtend.core.tests: there are only two of them, one for the Java files and one for the Xtend files.
• The XtendIncrementalBuilderPerformanceTest used to be in a package that ends with build so its directory it's excluded by the main .gitignore file; it took me a while to understand what was going on. To avoid problems in the future, I renamed its package to building.
• removed RuntimePerformanceSuite.java since it didn't seem to make sense

[szarnekow]

@LorenzoBettini Would Junit tags be option instead of splitting the project? See org.junit.jupiter.api.Tag

[LorenzoBettini]

@szarnekow I think JUnit tags, even with JUnit 4, would work.
It's just that those performance tests looked like they would fit that separate project.
However, I'll investigate JUnit 4 tags. I would keep a few commits of this PR that are more related to clean-up.

[LorenzoBettini]

@szarnekow from my understanding JUnit 4 does not provide tags: it provides categories that require proper suite classes. In fact, Eclipse allows for tag selection/exclusion only for JUnit 5.
Thus, for JUnit 4 this would not help.
I'd also avoid switching to JUnit 5 which gave a few headaches in the past.
What do you think?

[szarnekow]

Sadness. Ok, I'm fine with a separate test project or with a separate source folder for the performance tests. Whatever you prefer.

[LorenzoBettini]

@szarnekow I had also thought about a separate source folder in xtend.core.tests but that would work seamlessly for Java files: the two performance tests are Xtend files, so they are generated into xtend-gen together with the other ones, so the launch configuration could not distinguish them.

[LorenzoBettini]

I spotted other performance tests also in xtend.ide.tests; I'd like to move them as well to the project performance.tests

[szarnekow]

I'd like to move them as well

Please do