[build, ios] Add canary Xcode 10 CircleCI job

[friedbunny]

Fixes #12756. Updating to Xcode 10 proved to be slightly troublesome:

• 5882c1a bumped most every expectation wait timeout to 5 seconds, which seems to satisfy CircleCI. Tests always ran successfully locally, but the original timeouts were overrun without any apparent pattern on CI (and we may yet need to bump further). Cause is unknown.
• Foundation in Xcode 10 incorrectly classifies the word "optional" as a verb.

Let’s keep this build optional and let it run alongside the current Xcode 9.4.1 builds and, if it proves reliable, we can migrate the rest of the builds next week before ios-v4.5.0-alpha.2. We should do our best to publish ios-v4.5.0 with Xcode 10.

/cc @mapbox/maps-ios

[friedbunny]

@fabian-guerra Looks like this is failing since I rebased to include #12791:

MGLOfflineStorageTests
  testAddFileContent, ((error) == nil) failed: "Error Domain=NSCocoaErrorDomain Code=4 "The file “barcelona.db” doesn’t exist." UserInfo={NSFilePath=/Users/distiller/Library/Developer/CoreSimulator/Devices/5B49D0FB-D8AE-415B-9DCF-74CB9419D69F/data/Documents/barcelona.db, NSUnderlyingError=0x7fb97cd117a0 {Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"}}" - Changing the file's permissions:/Users/distiller/Library/Developer/CoreSimulator/Devices/5B49D0FB-D8AE-415B-9DCF-74CB9419D69F/data/Documents/barcelona.db should not return an error.
  /Users/distiller/project/platform/darwin/test/MGLOfflineStorageTests.mm:306

        [fileManager setAttributes:atributes ofItemAtPath:filePath error:&error];
        XCTAssertNil(error, @"Changing the file's permissions:%@ should not return an error.", filePath);
        error = nil;

  testAddFileContent, ((fileSize) equal to (dabaseFileSize)) failed: ("0") is not equal to ("32391168") - The dabase file size must be: 0x1.fffb879b794p-1028ctual size:
  /Users/distiller/project/platform/darwin/test/MGLOfflineStorageTests.mm:314

        // Merging databases creates an empty file if the file does not exist at the given path.
        XCTAssertEqual(fileSize, dabaseFileSize, @"The dabase file size must be:%l actual size:%l", dabaseFileSize, fileSize);
        
  testAddFileContent, failed: caught "NSInvalidArgumentException", "The file path: file:///Users/distiller/Library/Developer/CoreSimulator/Devices/5B49D0FB-D8AE-415B-9DCF-74CB9419D69F/data/Documents/barcelona.db must be writable"
  /Users/distiller/project/platform/darwin/test/MGLOfflineStorageTests.mm:326

        MGLOfflineStorage *os = [MGLOfflineStorage sharedOfflineStorage];
        [os addContentsOfFile:filePath withCompletionHandler:^(NSURL *fileURL, NSArray<MGLOfflinePack *> * _Nullable packs, NSError * _Nullable error) {
            XCTAssertNotNil(fileURL, @"The fileURL should not be nil.");

[friedbunny]

Even after #12913, I’m still seeing the new offline tests fail:

MGLOfflineStorageTests
  testAddFileContent, Asynchronous wait failed: Exceeded timeout of 2 seconds, with unfulfilled expectations: "add database content completion handler", "Expect change to 'packs' of <MGLOfflineStorage: 0x7fbb8a5b2a90>".
  /Users/distiller/project/platform/darwin/test/MGLOfflineStorageTests.mm:340

        }];
        [self waitForExpectationsWithTimeout:2 handler:nil];
        // Depending on the database it may update or add a pack. For this case specifically the offline database adds one pack.

  testAddFileContent, (([MGLOfflineStorage sharedOfflineStorage].packs.count) equal to (countOfPacks + 1)) failed: ("0") is not equal to ("1") - Adding contents of barcelona.db should add one pack.
  /Users/distiller/project/platform/darwin/test/MGLOfflineStorageTests.mm:342

        // Depending on the database it may update or add a pack. For this case specifically the offline database adds one pack.
        XCTAssertEqual([MGLOfflineStorage sharedOfflineStorage].packs.count, countOfPacks + 1, @"Adding contents of barcelona.db should add one pack.");
    }

This is happening on Xcode 9.4.1 and 10, so I’m starting to think this is a CircleCI performance problem. I’ll try bumping the timeout here (e566bfa) and rerunning.

[friedbunny]

@mapbox/maps-ios If tests pass again this time (after squashing), I’d like to go ahead and merge this.

[fabian-guerra]

😂thank you for catching this.