Disable TestURLSession.test_taskTimeout

[ianpartridge]

This is failing intermittently on Ubuntu 18.04 so disable it for now.

[ianpartridge]

@swift-ci please test

[shahmishal]

I think we also have to disable

TestURLSession.test_verifyHttpAdditionalHeaders

TestFoundation/TestURLSession.swift:254: error: TestURLSession.test_verifyRequestHeaders : XCTAssertNotNil failed - 
TestFoundation/TestURLSession.swift:255: error: TestURLSession.test_verifyRequestHeaders : XCTAssertNil failed: "URLError(_nsError: The operation could not be completed)" - error = URLError(_nsError: The operation could not be completed)
Test Case 'TestURLSession.test_verifyRequestHeaders' failed (5.008 seconds)
Test Case 'TestURLSession.test_verifyHttpAdditionalHeaders' started at 2018-06-13 18:24:39.342
TestFoundation/TestURLSession.swift:281: error: TestURLSession.test_verifyHttpAdditionalHeaders : XCTAssertNotNil failed - 
TestFoundation/TestURLSession.swift:282: error: TestURLSession.test_verifyHttpAdditionalHeaders : XCTAssertNil failed: "URLError(_nsError: The operation could not be completed)" - error = URLError(_nsError: The operation could not be completed)
Fatal error: Unexpectedly found nil while unwrapping an Optional value: file TestFoundation/TestURLSession.swift, line 283
Current stack trace:
0    libswiftCore.so                    0x00007f60c8336c50 _swift_stdlib_reportFatalErrorInFile + 215
1    libswiftCore.so                    0x00007f60c825cc74 <unavailable> + 3996788
2    libswiftCore.so                    0x00007f60c8014d80 <unavailable> + 1604992
3    libswiftCore.so                    0x00007f60c801498f <unavailable> + 1603983
4    libswiftCore.so                    0x00007f60c8012e6f <unavailable> + 1597039
5    libswiftCore.so                    0x00007f60c8012a60 _fatalErrorMessage(_:_:file:line:flags:) + 44
6    TestFoundation                     0x0000562bbc27c10d <unavailable> + 5742861
7    TestFoundation                     0x0000562bbc2911fc <unavailable> + 5829116
8    libFoundation.so                   0x00007f60c7538c82 <unavailable> + 7052418
9    libFoundation.so                   0x00007f60c73e96e0 BlockOperation.main() + 143
10   libFoundation.so                   0x00007f60c73e7a00 Operation.start() + 230
11   libFoundation.so                   0x00007f60c73eb0e0 OperationQueue._runOperation() + 267
12   libFoundation.so                   0x00007f60c73f2f27 <unavailable> + 5717799
13   libFoundation.so                   0x00007f60c73e7d80 <unavailable> + 5672320
14   libdispatch.so                     0x00007f60c85ed085 <unavailable> + 360581
15   libdispatch.so                     0x00007f60c85eafaa <unavailable> + 352170
16   libdispatch.so                     0x00007f60c85eb6ad <unavailable> + 353965
17   libdispatch.so                     0x00007f60c85edd6a <unavailable> + 363882
18   libpthread.so.0                    0x00007f60c7c756db <unavailable> + 30427
19   libc.so.6                          0x00007f60c382a850 clone + 63

[spevans]

There's actually a couple of URLSession tests that fail on 18.04. Im currently investigating them but don't have an ETA for a fix yet.