gitutils: add diffStrings, diffFiles, and use it in testament to compare expected vs gotten (#17892)

* gitutils: add diffStrings, diffFiles, and use it in testament to compare expected vs gotten
* refactor with createTempDir
* cleanup
* refacotr
* PRTEMP fake test spec changes to show effect of diffStrings
* add runnableExamples for experimental/diff + cross-reference with gitutils
* Revert "PRTEMP fake test spec changes to show effect of diffStrings"

This reverts commit 57dc8d642dce6c1127c98b7cbc9edbfe747d4047.

testament/categories.nim

@@ -665,8 +665,8 @@ proc runJoinedTest(r: var TResults, cat: Category, testsDir: string, options: st
 
   if buf != outputExpected:
     writeFile(outputExceptedFile, outputExpected)
-    discard execShellCmd("diff -uNdr $1 $2" % [outputExceptedFile, outputGottenFile])
-    echo failString & "megatest output different!"
+    echo diffFiles(outputGottenFile, outputExceptedFile).output
+    echo failString & "megatest output different, see $1 vs $2" % [outputGottenFile, outputExceptedFile]
     # outputGottenFile, outputExceptedFile not removed on purpose for debugging.
     quit 1
   else:

testament/testament.nim

@@ -17,6 +17,7 @@ from std/sugar import dup
 import compiler/nodejs
 import lib/stdtest/testutils
 from lib/stdtest/specialpaths import splitTestFile
+from std/private/gitutils import diffStrings
 
 proc trimUnitSep(x: var string) =
   let L = x.len
@@ -307,7 +308,7 @@ proc addResult(r: var TResults, test: TTest, target: TTarget,
       maybeStyledEcho styleBright, expected, "\n"
       maybeStyledEcho fgYellow, "Gotten:"
       maybeStyledEcho styleBright, given, "\n"
-
+      echo diffStrings(expected, given).output
 
   if backendLogging and (isAppVeyor or isAzure):
     let (outcome, msg) =