[fine] Tweak syntax for assertions to make me happier

fine/tests/README.md

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+# Snapshot Tests
+
+The `.fine` files in this directory and its descendants are processed
+by `build.rs` into a series of snapshot-tests. The various test
+assertions are specified by `@` directives in comments in the file.
+
+e.g., a test might look like this:
+
+```fine
+// @concrete:
+// | File
+// |   ExpressionStatement
+// |     BinaryExpression
+// |       BinaryExpression
+// |         LiteralExpression
+// |           Number:'"1"'
+// |         Star:'"*"'
+// |         LiteralExpression
+// |           Number:'"2"'
+// |       Plus:'"+"'
+// |       BinaryExpression
+// |         UnaryExpression
+// |           Minus:'"-"'
+// |           LiteralExpression
+// |             Number:'"3"'
+// |         Star:'"*"'
+// |         LiteralExpression
+// |           Number:'"4"'
+// |     Semicolon:'";"'
+//
+1 * 2 + -3 * 4;
+
+// @type: 532 f64
+```
+
+## Assertions
+
+The various assertions are as follows:
+
+- The `// @concrete:` assertion says that the following lines
+  (prefixed with `// | `, as above) describe the concrete syntax tree
+  of the file after parsing.
+
+  e.g.:
+
+  ```fine
+  // @concrete:
+  // | File
+  // |   ExpressionStatement
+  // |     LiteralExpression
+  // |       String:'"\"Hello world\""'
+  // |     Semicolon:'";"'
+  //
+  "Hello world";
+  ```
+
+- The `// @type:` assertion says that the type of the tree at the
+  given point will match the given type. `@type` assertions usually go
+  after the contents of the file to make the probe points more stable
+  in the face of new assertions and whatnot.
+
+  e.g.:
+
+  ```fine
+  "Hello world!"
+  // @type: 2 string
+  ```
+
+- The `// @type-error:` assertion says that the type of the tree at
+  the given point should be an error, and that the error message
+  provided should be among the generated errors. (Like `@type`, these
+  usually go after the code, for stability.)
+
+  e.g.:
+
+  ```fine
+  - "twenty five";
+  // @type-error: 0 cannot apply unary operator '-' to value of type string
+  ```