test: give better error messages for `read_text`

tests/test_read_text.py

 def test_read_text():
     from src.enhanced_occurrence_frequencies import read_text
 
-    assert read_text("tests/sample.txt") == "this is a sample text file\n"
-    assert read_text("tests/empty.txt") == ""
-    assert read_text("tests/multiline.txt") == "i\nam\na\nmultiline\ntext\n"
-    assert read_text("tests/symbols.txt") == "this is a (sample) text, containing (a lot of) punctuation!!\n"
+    for (file, expected, msg) in [
+        ("tests/sample.txt", "this is a sample text file\n", None),
+        (
+            "tests/empty.txt",
+            "",
+            "`read_text` should give an empty string for an empty file",
+        ),
+        (
+            "tests/multiline.txt",
+            "i\nam\na\nmultiline\ntext\n",
+            "`read_text` should handle newlines",
+        ),
+        (
+            "tests/symbols.txt",
+            "this is a (sample) text, containing (a lot of) punctuation!!\n",
+            "`read_text` should only make the text lower case, not remove symbols",
+        ),
+    ]:
+        if msg is not None:
+            assert read_text(file) == expected, msg
+        else:
+            assert read_text(file) == expected