Add xUnit1070/xUnit1071: cultured tests cannot have null or duplicate cultures

[meziantou]

Fixes xunit/xunit#3632.

Adds two rules for [CulturedFact] and [CulturedTheory]:

ID	Severity	Reported when
xUnit1070 – Cultured test methods cannot have null cultures	Error	A culture is null, default, default(string), or a null const string
xUnit1071 – Cultured test methods should not have duplicate cultures	Warning	A culture is listed more than once (Culture '{0}' is duplicated.)

A null culture breaks the test at runtime. A duplicate only runs the same test twice, so it's a warning.

What changed

• The rules are new diagnostic IDs in the same analyzer as xUnit1060, which does the same argument analysis. I renamed the class from CulturedTestMustHaveAtLeastOneCulture to CulturedTestCultureValidation because it now covers three rules. This follows ConditionalSkipPropertyValidation (xUnit1054/1055).
• It uses the IOperation API and handles both array creation (new[] { ... }) and collection expressions ([...]). Each bad element is reported at its own location. For duplicates, the second and later occurrences are reported.
• When the culture list is empty, only xUnit1060 is reported.

Notes for reviewers

• Case: duplicates are compared ignoring case ("en-US" and "en-us" are the same culture).
• Empty string: "" (invariant culture) is allowed.
• Not covered: invalid culture names such as "xx-BOGUS". Whether a name is valid depends on the OS culture data (ICU), so the analyzer can't check it reliably.
• xUnit1060 change: a null cultures array ([CulturedFact(null)], default, (string[])null) is now reported as xUnit1060. At runtime it fails with the same "did not provide any cultures" error as an empty array.
• Tests: there's one test file per ID, each with a single large V3_only test. Because the analyzer now reports more than one ID, the X1060 test markup names its ID ({|xUnit1060:...|}).
• Docs: the rule pages for xUnit1070 and xUnit1071 still need to be added in the xunit.net site repo.

[bradwilson]

Thanks!

[bradwilson]

Available in 2.2.0-pre.21 https://xunit.net/docs/using-ci-builds