replace `assertStringEqual` by version checking for content and len

Now check for content *and* length of the strings in
`tserialization`. Also adds a test case for the case with which the
old version would have failed (one string longer than the other, thus
being different).

This version tries to keep the error reporting mechanism the same as
the old version.

test/tserialization.nim

@@ -5,7 +5,7 @@
 #    distribution, for details about the copyright.
 
 import "../yaml"
-import unittest, strutils, tables, times, math
+import unittest, strutils, tables, times, math, os
 
 type
   MyTuple = tuple
@@ -96,13 +96,31 @@ proc constructObject*(s: var YamlStream, c: ConstructionContext,
   constructScalarItem(s, item, BetterInt):
     result = BetterInt(parseBiggestInt(item.scalarContent) + 1)
 
-template assertStringEqual(expected, actual: string) =
-  for i in countup(0, min(expected.len, actual.len) - 1):
+template assertStringEqual(expected, actual: string, file = stdout) =
+  if expected != actual:
+    # if they are unequal, walk through the strings and check each
+    # character for a better error message
+    if expected.len != actual.len:
+      file.write "Expected and actual string's length differs.\n"
+      file.write "Expected length: ", expected.len, "\n"
+      file.write "Actual length: ", actual.len, "\n"
+    # check length up to smaller of the two strings
+    for i in countup(0, min(expected.high, actual.high)):
       if expected[i] != actual[i]:
-      echo "string mismatch at character #", i, "(expected:\'",
-            expected[i], "\', was \'", actual[i], "\'):"
-      echo "expected:\n", expected, "\nactual:\n", actual
+        file.write "string mismatch at character #", i, "(expected:\'",
+         expected[i], "\', was \'", actual[i], "\'):\n"
+        file.write "expected:\n", expected, "\nactual:\n", actual, "\n"
         assert(false)
+    # if we haven't raised an assertion error here, the problem is that
+    # one string is longer than the other
+    let minInd = min(expected.len, actual.len) # len instead of high to continue
+                                               # after shorter string
+    if expected.high > actual.high:
+      file.write "Expected continues with: '", expected[minInd .. ^1], "'"
+      assert false
+    else:
+      file.write "Actual continues with: '", actual[minInd .. ^1], "'"
+      assert false
 
 template expectConstructionError(li, co: int, message: string, body: typed) =
   try:
@@ -124,6 +142,19 @@ proc newNode(v: string): ref Node =
 let blockOnly = defineOptions(style=psBlockOnly)
 
 suite "Serialization":
+  test "Internal string assert":
+    let s1 = "foo"
+    let s2 = "foobar"
+    let fname = "dump.txt"
+    let f: File = open(fname, fmWrite)
+    try:
+      assertStringEqual(s1, s2, file=f)
+    except AssertionError:
+      discard
+    finally:
+      f.close()
+      removeFile(fname)
+
   test "Load integer without fixed length":
     var input = "-4247"
     var result: int