Fix typos

Author: Chris Williams

Sources/XCTest/XCTAssert.swift

@@ -129,7 +129,7 @@ private func _XCTEvaluateAssertion(_ assertion: _XCTAssertion, message: @autoclo
 ///   }
 ///   ```
 ///
-///  Calling this assertion will cause XCTest to report the failure occured
+///  Calling this assertion will cause XCTest to report the failure occurred
 ///  in the file where `AssertEmpty()` is defined, and on the line where
 ///  `XCTAssertEqual` is called from within that function:
 ///

Sources/XCTest/XCTestCase.swift

@@ -121,7 +121,7 @@ public class XCTestCase: XCTest {
 
         // FIXME: Apple XCTest does not throw a fatal error and crash the test
         //        process, it merely prevents the remainder of a testClosure
-        //        from execting after it's been determined that it has already
+        //        from expecting after it's been determined that it has already
         //        failed. The following behavior is incorrect.
         // FIXME: No regression tests exist for this feature. We may break it
         //        without ever realizing.

Sources/XCTest/XCTestCaseSuite.swift

@@ -8,7 +8,7 @@
 //
 //
 //  XCTestCaseSuite.swift
-//  A test suite associated with a paticular test case class.
+//  A test suite associated with a particular test case class.
 //
 
 #if os(Linux) || os(FreeBSD)