Create the coverage directory before writing

Odd that this failed - we should make the directory within the test
suite - but let's see if this helps.

src/test/java/dan200/computercraft/core/ComputerTestDelegate.java

@@ -333,6 +333,7 @@ public class ComputerTestDelegate
 
         if( finishedWith != null )
         {
+            REPORT_PATH.getParentFile().mkdirs();
             try( BufferedWriter writer = Files.newBufferedWriter( REPORT_PATH.toPath() ) )
             {
                 new LuaCoverage( finishedWith ).write( writer );