added tests for hidden-compiler

Author: rssh

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -177,6 +177,7 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/matchable.scala", defaultOptions.and("-Xfatal-warnings", "-source", "future")),
       compileFile("tests/neg-custom-args/i7314.scala", defaultOptions.and("-Xfatal-warnings", "-source", "future")),
       compileFile("tests/neg-custom-args/feature-shadowing.scala", defaultOptions.and("-Xfatal-warnings", "-feature")),
+      compileDir("tests/neg-custom-args/hidden-type-errors",  defaultOptions.and("-explain")),
     ).checkExpectedErrors()
   }