Add test for basename

test/sourcemaps/samples/sourcemap-basename/_config.js

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+import { magic_string_bundle } from '../../helpers';
+
+export const component_filepath = 'src/input.svelte';
+
+export const component_file_basename = 'input.svelte';
+
+// as output by external tool for src/external_code.css (relative to src/input.svelte)
+export const external_relative_filename = 'external_code.css';
+
+const external_code = `
+span {
+	--external_code-var: 1px;
+}
+`;
+
+export default {
+	css_map_sources: [external_relative_filename],
+	js_map_sources: [],
+	preprocess: [
+		{
+			style: ({ content, filename }) => {
+				const external =`/* Filename from preprocess: ${filename} */` + external_code;
+				return magic_string_bundle([
+					{ code: external, filename: external_relative_filename },
+					{ code: content, filename }
+				]);
+			}
+		}
+	],
+	options: {
+		filename: component_filepath
+	}
+};

test/sourcemaps/samples/sourcemap-basename/input.svelte

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+<style src="external_code.css"></style>
+
+<span>Hello world</span>

test/sourcemaps/samples/sourcemap-basename/test.js

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+import { component_filepath, component_file_basename, external_relative_filename } from './_config';
+
+export function test({ assert, preprocessed }) {
+	assert.notEqual(
+		preprocessed.locate(`/* Filename from preprocess: ${component_filepath} */`),
+		undefined,
+		'Preprocessor should receive same value for filename as passed to preprocess function'
+	);
+	
+	assert.deepEqual(
+		preprocessed.map.sources.slice().sort(),
+		[external_relative_filename, component_file_basename].sort(),
+		'Preprocessed map should contain sources relative to filepath'
+	);
+}