Add errors assertion (#444)

This is a super common assertion and something I wanted to use in a
project I was working on so figure'd I would add it.

Not sure what the larger vision is for testing API, but hopefully this
fits somewhere.

Can also add an optional `expectedError` param

Author: wmccrthy

examples/testing.luau

@@ -50,6 +50,21 @@ test.suite("MySuite", function(suite)
 		assert.tableeq(table1, table2)
 		assert.tableeq(table1, { a = 1 })
 	end)
+
+	suite:case("expect_error", function(assert)
+		local function foo(v: boolean)
+			if v then
+				error("error")
+			end
+		end
+
+		assert.errors(function() -- demonstrates success (callback throws err)
+			foo(true)
+		end)
+		assert.errors(function() -- demonstrates failure (callback doesn't throw)
+			foo(false)
+		end)
+	end)
 end)
 
 test.run()

lute/std/libs/assert.luau

@@ -12,6 +12,13 @@ function assertions.neq<T>(lhs: T, rhs: T)
 	end
 end
 
+function assertions.errors(callback: () -> ())
+	local success = pcall(callback)
+	if success then
+		error({ msg = `{callback} did not throw error.` })
+	end
+end
+
 -- Helper to format values for error messages
 local function formatValue(val: any): string
 	if type(val) == "nil" then