feat(tests): implement a way to express that tests can fail

crates/aiken-lang/src/parser/lexer.rs

@@ -115,6 +115,7 @@ pub fn lexer() -> impl Parser<char, Vec<(Token, Span)>, Error = ParseError> {
         "todo" => Token::Todo,
         "type" => Token::Type,
         "when" => Token::When,
+        "fail" => Token::Fail,
         "validator" => Token::Validator,
         _ => {
             if s.chars().next().map_or(false, |c| c.is_uppercase()) {

crates/aiken-lang/src/parser/token.rs

@@ -80,6 +80,7 @@ pub enum Token {
     When,
     Trace,
     Validator,
+    Fail,
 }
 
 impl fmt::Display for Token {
@@ -164,6 +165,7 @@ impl fmt::Display for Token {
             Token::Test => "test",
             Token::ErrorTerm => "error",
             Token::Validator => "validator",
+            Token::Fail => "fail",
         };
         write!(f, "\"{s}\"")
     }