compiler: add interface IR test

This is important as golden test output and to verify that the output is
correct. Later improvements and bug fixes are clearly visible in the IR,
and unintentional changes will also be immediately spotted.

compiler/compiler_test.go

@@ -61,6 +61,7 @@ func TestCompiler(t *testing.T) {
 		"slice.go",
 		"string.go",
 		"float.go",
+		"interface.go",
 	}
 
 	for _, testCase := range tests {