Move tests of NIOCore types from NIOPosixTests to NIOCoreTests

[simonjbeaumont]

Motivation:

As described in #2016, there are many tests in NIOPosixTests for types that have since moved from NIOPosix to NIOCore.

Modifications:

Move the tests from NIOPosixTests that do not import NIOPosix into NIOCoreTests

Result:

Tests for types defined in the NIOCore module are more likely to be located in NIOCoreTests test target.

Resolves #2016.

[Lukasa]

Nice one, solid change.

[Lukasa]

You've got a few CI failures to fix.

[simonjbeaumont]

You've got a few CI failures to fix.

20:53:16 [196/272] Compiling NIOCoreTests LinuxTest+XCTest.swift
20:53:16 /code/Tests/NIOCoreTests/LinuxTest.swift:33:13: error: cannot find 'withTemporaryFile' in scope
20:53:16             withTemporaryFile(content: quota) { (_, quotaPath) -> Void in

... LinuxTest. Makes sense why this passed locally :) Will fix.

[Lukasa]

Looks like some helper functions are missing.

[simonjbeaumont]

@Lukasa I think this is all resolved now. The 5.7 and nightly pipelines are failing with some allocation counts. Presumably that's not related.

[Lukasa]

Seems unlikely, yeah.