[XCTestMain] Don't XCTPrint arrays of strings
[XCTestMain] Don't XCTPrint arrays of strings

`XCTPrint()` takes a variadic argument of items to print, mirroring the
API of `print()` itself. Unfortunately, passing this variadic argument
directly to `print()` causes an array object to be printed. For example:

```swift
func XCTPrint(items: Any...) {
    print(items)
}

XCTPrint("Hello!") // Prints '["Hello!"]\n'
```

As a result of this behavior, swift-corelibs-xctest currently prints
out test output such as the following:

```
["Test Case \'ErrorHandling.test_shouldButDoesNotThrowErrorInAssertion\' failed (0.0 seconds)."]
["Test Case \'ErrorHandling.test_shouldThrowErrorInAssertion\' started."]
```

This is different from the expected output in `Tests/Functional`, which
is causing the test suite to fail.

Instead of mirroring the `print()` API, have `XCTPrint()` simply take a
string object to print. This has the added benefit of being simpler--we
can add the variadic arguments and the line separator parameter back if
we ever need them.

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