* 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
Fix#6588
Note for the test: parse error at the end of the file are actually
reported at offset 0. Use a trick to add more `^` at the beginning to
mean that.
(Offset 0 is actually offset 1 as the column starts at 1)
