testsuite: Implement `isInTreeCompiler` in a more robust way

Just a small refactoring to avoid redundantly specifying the same
strings in two different places.

hadrian/src/Oracles/TestSettings.hs

@@ -111,7 +111,7 @@ getCompilerPath "stage-cabal" = do
 getCompilerPath compiler = pure compiler
 
 isInTreeCompiler :: String -> Bool
-isInTreeCompiler c = c `elem` ["stage1","stage2","stage3"]
+isInTreeCompiler c = isJust (stageOfTestCompiler c)
 
 -- | Get the full path to the given program.
 fullPath :: Stage -> Package -> Action FilePath

hadrian/src/Rules/Test.hs

@@ -304,7 +304,7 @@ testRules = do
 -- We should have built them already by this point, but
 isOkToBuild :: TestArgs -> String -> Bool
 isOkToBuild args target
-   = isJust (stageOf (testCompiler args))
+   = isInTreeCompiler (testCompiler args)
   || testHasInTreeFiles args
   || target `elem` map cp_target checkPrograms