Update docs for decoding stdout and stderr from exit tests

[simonjbeaumont]

Motivation:

The API documentation for ExitTest.Result.standardOutputContent and ExitTest.Result.standardErrorContent has advice on how to construct a string from their contents. It recommends using String.init(validatingCString:), which has two problems:

1. You cannot just pass it in since these properties are [UInt8] and String.init(validatingCString:) takes sequences whose element is CChar (aka Int8):

_ = String(validatingCString: result.standardErrorContent)  // error: Cannot convert value of type '[UInt8]' to expected argument type '[CChar]' (aka 'Array<Int8>')

2. Even if you had the sequence of the correct element type, the matching initializer is now deprecated:

_ = String(validatingCString: result.standardErrorContent.map(Int8.init))  // warning: 'init(validatingCString:)' is deprecated: Use String(validating: array, as: UTF8.self) instead, after truncating the null termination.

So, in order to get this working without warnings, adopters will likely want this:

try #require(String(validating: result.standardErrorContent, as: UTF8.self))

Modifications:

• Update the API documentation to advise using String(validating: array, as: UTF8.self).

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

Thanks! Makes sense to me, but I'll let @grynspan weigh in first

[grynspan]

Makes sense to me. I think I originally wrote this documentation before that initializer was added.