Refactor parser fuzz testing

crates/ra_syntax/src/fuzz.rs

@@ -0,0 +1,12 @@
+use crate::{SourceFile, validation, AstNode};
+
+fn check_file_invariants(file: &SourceFile) {
+    let root = file.syntax();
+    validation::validate_block_structure(root);
+    let _ = file.errors();
+}
+
+pub fn check_parser(text: &str) {
+    let file = SourceFile::parse(text);
+    check_file_invariants(&file);
+}