{Mutable,}Dictionary: minor changes

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
SyntaxProcessor: fix a misplaced replacement

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
AbstractSyntaxChecker: use an enum-based message

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
ProcessingMessageAssert: prepare for future tests

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>