[Testament] Extend and document message testing aids (#19996)

* [Testament] Extend and document message testing aids

* Enable inline msgs when not reject action.
Eliminates the pain of changing the line and column numbers in `nimout`
or `output` while making changes to the test.
* Enable using inline msgs and nimout together.
Allows ease of inline msgs for the test as well as testing msgs from
other modules.
* Add path separator and test filename variable interpolation in
msgs.
Eases handling path separators in the msgs.
* Add some documentation.

* Fixed lots of broken tests

* Fixed more broken tests

* Support multiple inline messages per a line

* Fix a broken test

* Revert variable substitution in `output`

* Remove uneeded params

* Update doc/testament.md

Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>

* Update testament/specs.nim

Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>

* Update testament/specs.nim

Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>

* Fix indentation

Co-authored-by: quantimnot <quantimnot@users.noreply.github.com>
Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>

tests/pragmas/tvar_macro.nim

@@ -85,7 +85,7 @@ block: # with other pragmas
   let importedFooBar {.importc: "exportedFooBar", nodecl.}: set[char]
 
   doAssert importedFooBar == fooBar #[tt.Warning
-                            ^ fooBar is deprecated
+                             ^ fooBar is deprecated
   ]#