Test cleanup to lay groundwork for executing test programs

Rename test output from `-output` to `.output`, because DOS short filenames are hostile even when you think you're not using them, a gift that keeps on giving decades later
Delete duplicate `.cpp2` files in the `test-results` folder after test run is done

Author: hsutter

regression-tests/run-tests.bat

@@ -3,12 +3,14 @@ copy ..\*.cpp2 .
 set count=0
 for %%f in (mixed-*.cpp2) do (
     echo Starting cppfront.exe %%f
-    C:\GitHub\cppfront\x64\Debug\cppfront.exe %%f > %%f-output 2>&1
+    C:\GitHub\cppfront\x64\Debug\cppfront.exe %%f > %%f.output 2>&1
+    del %%f
     set /a count+=1
 )
 for %%f in (pure2-*.cpp2) do (
     echo Starting cppfront.exe %%f -p
-    C:\GitHub\cppfront\x64\Debug\cppfront.exe %%f -p > %%f-output 2>&1
+    C:\GitHub\cppfront\x64\Debug\cppfront.exe %%f -p > %%f.output 2>&1
+    del %%f
     set /a count+=1
 )
 echo Done: %count% tests