[SR-10505] Disabling TestNSString#test_FromContentsOfURL as being flaky

[xedin]

Previous ID	SR-10505
Radar	None
Original Reporter	@xedin
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: 83ba7d0329182eea01b2768a14a1e7ca

Issue Description:

Multiple CI builds failing with following error:

/home/buildnode/jenkins/workspace/oss-swift-incremental-RA-linux-ubuntu-16_04/swift-corelibs-foundation/TestFoundation/TestNSString.swift:392: error: TestNSString.test_FromContentsOfURL : XCTAssertNotNil failed - 
19:28:37 /home/buildnode/jenkins/workspace/oss-swift-incremental-RA-linux-ubuntu-16_04/swift-corelibs-foundation/TestFoundation/TestNSString.swift:393: error: TestNSString.test_FromContentsOfURL : XCTAssertEqual failed: ("0") is not equal to ("5") - 
19:28:37 Test Case 'TestNSString.test_FromContentsOfURL' failed (0.002 seconds)

Example: https://ci.swift.org/job/oss-swift-incremental-RA-linux-ubuntu-16_04/8262/console

[belkadan]

cc @millenomi