Add ability to run only one(focused) example

[PaulTaykalo]

Adds helper method, that allows marking only one example to be run when running tests

Sometimes, when specific rule example is debugged, it is really helpful to run one 'test'/'example'.
This PR adds ability to mark Examples that need to be checked when running tests

i.e.

nonTriggeringExamples: [
    Example("let x: [Int]"),
    Example("let x: [Int: String]").focused()   // only this one will be run in tests
],
triggeringExamples: [
    Example("let x: ↓Array<String>"),
    Example("let x: ↓Dictionary<Int, String>")
]

In case if multiple examples marked as 'focused', then all of them will be checked when running tests.

This PR is inspired by the same functionality in other testing frameworks:
https://github.com/Quick/Quick/blob/72e9afa98f00c719b16c359d67d95366bad0a460/Documentation/en-us/QuickExamplesAndGroups.md#temporarily-running-a-subset-of-focused-examples

[SwiftLintBot]

	12 Messages
📖	Linting Aerial with this PR took 0.98s vs 0.97s on master (1% slower)
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[codecov-commenter]

Codecov Report

Merging #3911 (fd2d9fe) into master (702b36a) will decrease coverage by 0.05%.
The diff coverage is 60.71%.

@@            Coverage Diff             @@
##           master    #3911      +/-   ##
==========================================
- Coverage   92.61%   92.56%   -0.06%     
==========================================
  Files         436      436              
  Lines       21964    21990      +26     
==========================================
+ Hits        20343    20355      +12     
- Misses       1621     1635      +14     

Impacted Files	Coverage Δ
Source/SwiftLintFramework/Models/Example.swift	86.20% <20.00%> (-13.80%)	⬇️
Tests/SwiftLintFrameworkTests/TestHelpers.swift	79.08% <69.56%> (-0.80%)	⬇️
Source/SwiftLintFramework/Models/Linter.swift	79.09% <0.00%> (-1.70%)	⬇️

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[SimplyDanny]

I like the idea of focusing on specific examples. Commenting out all but the relevant examples finds an end with this.

I wonder, though, whether it can be made sure that code with focused examples is not checked in. There could be a custom SwiftLint rule which checks for .focused() in rule code. Or the build breaks somehow if the function is called while running on CI.

[jpsim]

Thanks, this will be helpful!

[jpsim]

Ah yes Danny's suggestion would be good to consider. I'd be ok if we had a custom rule preventing this from being checked in.

[PaulTaykalo]

@jpsim is there any additional env that is set on CI?

[PaulTaykalo]

@jpsim how about allowing this functionality under #if DEBUG only ?

[PaulTaykalo]

@jpsim @SimplyDanny
added DISABLE_FOCUSED_EXAMPLES conditional, which will be enabled only on CI
If this conditional enabled, focused examples will fail