added comment about frames in test files

src/RawSubFile.test.cpp

@@ -39,6 +39,12 @@ TEST_CASE("Read frames directly from a RawSubFile", "[.files]"){
 }
 
 TEST_CASE("Read frames directly from a RawSubFile starting at the second file", "[.files]"){
+    // we know this file has 10 frames with frame numbers 1 to 10
+    // f0 1,2,3
+    // f1 4,5,6 <-- starting here
+    // f2 7,8,9
+    // f3 10
+    
     auto fpath_raw = test_data_path() / "raw/jungfrau" / "jungfrau_single_d0_f1_0.raw";
     REQUIRE(std::filesystem::exists(fpath_raw));