syntax_tests: allow to update tests, and don't use a regexp (#8589)

* syntax_tests: allow to "bless" tests, and don't use a regexp

A regexp was used at the beginning because I thought we would want to
allow error to contains things that were not predictable or that would
often change. But this is not the case¹. It is better to actually test
for the full error message

¹ well actually it was the case for path, but there is another substitution to 
 `📂` for the manifest directory

* syntax_tests: Bless the tests

* syntax_tests: Manual adjust after bless

Because there used to be comments on the same line of the message which
bless don't support

* Fix error message with path on windows

 - The debug implementation of path make double slash, that's not what
   we want to show the user
 - normalize paths to use `/` so the test passes

internal/compiler/tests/syntax/elements/component_container.slint

@@ -9,6 +9,7 @@ export component Foo inherits Rectangle {
 }
 
 export Bar := Rectangle {
+//         ^warning{':=' to declare a component is deprecated. The new syntax declare components with 'component MyComponent {'. Read the documentation for more info}
     Foo { Text {} }
     ComponentContainer {
 //  ^error{ComponentContainers may not have children}