Fix must_use warnings

Various tests discard the return value from `format!`, which is
`must_use`. Make this discard explicit with `let _ = ...`.

Author: nicholasbishop

sbat/src/error.rs

@@ -69,8 +69,8 @@ mod tests {
 
         // For the rest, don't bother testing the specific error
         // messages, just ensure nothing panics.
-        format!("{}", ParseError::InvalidAscii);
-        format!("{}", ParseError::InvalidGeneration);
-        format!("{}", ParseError::TooFewFields);
+        let _ = format!("{}", ParseError::InvalidAscii);
+        let _ = format!("{}", ParseError::InvalidGeneration);
+        let _ = format!("{}", ParseError::TooFewFields);
     }
 }

sbat/src/revocation_section.rs

@@ -185,12 +185,12 @@ mod tests {
     fn test_error_display() {
         // Don't bother testing the specific error messages, just ensure
         // nothing panics.
-        format!("{}", RevocationSectionError::MissingVersion);
-        format!("{}", RevocationSectionError::InvalidVersion(1));
-        format!("{}", RevocationSectionError::MissingHeader);
-        format!("{}", RevocationSectionError::InvalidPreviousOffset(0));
-        format!("{}", RevocationSectionError::InvalidLatestOffset(0));
-        format!("{}", RevocationSectionError::MissingPreviousNull);
-        format!("{}", RevocationSectionError::MissingLatestNull);
+        let _ = format!("{}", RevocationSectionError::MissingVersion);
+        let _ = format!("{}", RevocationSectionError::InvalidVersion(1));
+        let _ = format!("{}", RevocationSectionError::MissingHeader);
+        let _ = format!("{}", RevocationSectionError::InvalidPreviousOffset(0));
+        let _ = format!("{}", RevocationSectionError::InvalidLatestOffset(0));
+        let _ = format!("{}", RevocationSectionError::MissingPreviousNull);
+        let _ = format!("{}", RevocationSectionError::MissingLatestNull);
     }
 }