fix: exclude tests with.npmignore (#1637)

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## PR Checklist

- [x] Addresses an existing open issue: fixes #1629 
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/create-typescript-app/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/create-typescript-app/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

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Doing this on a mobile phone on a train. It can never work, can it?

.npmignore

@@ -0,0 +1 @@
+*.test.*

src/steps/writing/creation/rootFiles.ts

@@ -25,6 +25,7 @@ export async function createRootFiles(options: Options) {
 				"node_modules/",
 			]),
 		}),
+		".npmignore": `*.test.*\n`,
 		".nvmrc": `20.12.2\n`,
 		".prettierignore": formatIgnoreFile([
 			...(options.excludeAllContributors ? [] : [".all-contributorsrc"]),