test(compiler): Abort test if a stream for a YAML file is in a bad state

The code in `loadEndToEndDesc` attempts to load the contents of a YAML
file without ensuring that the stream from which the file is read was
set up correctly. As a result, streams in a bad state, e.g., if the
file to be opened does not exist, simply result in an empty string for
the file contents. The test driver cannot distinguish this result from
a valid YAML file that is simply empty.

This patch causes `loadEndToEndDesc` to abort execution whenever a
file input stream could not be set up correctly.

compiler/tests/end_to_end_fixture/EndToEndFixture.cpp

@@ -284,6 +284,12 @@ LLVM_YAML_IS_DOCUMENT_LIST_VECTOR(EndToEndDesc)
 
 std::vector<EndToEndDesc> loadEndToEndDesc(std::string path) {
   std::ifstream file(path);
+
+  if (!file.good()) {
+    std::cerr << "Could not read yaml file: " << path << std::endl;
+    assert(false);
+  }
+
   std::string content((std::istreambuf_iterator<char>(file)),
                       (std::istreambuf_iterator<char>()));