[test] Fix double close in testCloseRace

Close the fd via the FileHandle rather than directly on the file
descriptor so that it does not get closed again when the pipe is
released. Fixes running test in parallel on Linux.

Author: benlangmuir

Tests/LanguageServerProtocolJSONRPCTests/ConnectionTests.swift

@@ -239,8 +239,7 @@ class ConnectionTests: XCTestCase {
         expectation.fulfill()
       })
 
-
-      close(to.fileHandleForWriting.fileDescriptor)
+      to.fileHandleForWriting.closeFile()
       // 100 us was chosen empirically to encourage races.
       usleep(100)
       conn.close()