feat: show the built-in test framework on the home page

[magnus-madsen]

The tooling comparison table further down the page claims that flix is its own test framework, but nothing on the page showed what a test actually looks like.

Adds a section between Termination Checking and First-class Datalog Constraints, text-left / code-right so the alternation across the page continues.

The example

use Assert.{assertEq, assertErr}

def divide(x: Int32, y: Int32): Result[String, Int32] =
    if (y == 0) Err("Division by zero") else Ok(x / y)

@Test
def testDivide01(): Unit \ Assert =
    assertEq(expected = Ok(5), divide(10, 2))

@Test
def testDivide02(): Unit \ Assert =
    assertErr(divide(10, 0))

It shows @Test, two assertions from the Assert module including the labelled expected argument, and the \ Assert effect — which lets the card tie testing back to the effect system the preceding twenty sections build up: asserting is an effect, so the test runner is just its handler.

Underneath the snippet is the output of flix test:

Running 2 tests...

   PASS  testDivide01 1.4ms
   PASS  testDivide02 0.3ms

Passed: 2, Failed: 0. Skipped: 0. Elapsed: 2.1ms.

Notes

The output is terminal text, not Flix, so it bypasses CodeSnippet and renders through a plain <pre> that reuses the component's frame — CodeSnippet only knows the Flix grammar and would colour the PASS lines as if they were code. No new CSS; overflow-x is set inline because Astro's <Code> ships that on the <pre> it renders and a bare one has nothing.

Timings are written with a decimal point. The book shows 1,4ms, which is the runner picking up a Danish locale.

Rebased onto #212 and #213; the diff is 62 insertions and no deletions. npm run check and npm run build both pass.

🤖 Generated with Claude Code