fix: correct test runner to fail on all unmet expected errors and refactor into Test class

The test runner previously short-circuited when a test expected an error and found at least one, causing it to miss other expected errors that didn't occur. This commit fixes the error pattern regex to exclude line-specific error expectations, removes specific error message strings from test files (since the runner doesn't validate them yet), and refactors the monolithic test function into a dedicated Test class with parse and run methods for better maintainability.

test/language/function/newline_in_expression_block.wren

@@ -1,5 +1,2 @@
-new Fn { IO.print("ok") // expect error
-}.call() // expect error
-
-// The second error is cascaded here. If it starts failing, just remove that
-// expectation.
+Fn.new { IO.print("ok") // expect error
+}.call()