[XCTestCase] Workaround SR-1129 using public var
[XCTestCase] Workaround SR-1129 using public var

`XCTestCase.name` works around a bug in the Linux Swift compiler by
declaring what should be a `private` variable as `public`.
`XCTAllExpectations` works around the same bug by using a global
variable. Unify the two by using the same workaround method: declare
the expectations variable as `public`.

yoonapps/swift-corelibs-xctest,apple/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest,apple/swift-corelibs-xctest