Turbopack build: Fix css-compilation tests (#68891)

## What?

No changes in behavior, just changes to the way the CSS is collected,
moving it from reading build directories to using the served pages
instead. Moved the snapshots into the tests using inline snapshots which
makes it much easier to compare them.

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Author: timneutkens

test/integration/css-fixtures/npm-import-nested/styles/global.css

@@ -1 +1 @@
-@import '~example/test.css';
+@import 'example/test.css';

test/integration/css-fixtures/npm-import/styles/global.css

@@ -1 +1 @@
-@import '~nprogress/nprogress.css';
+@import 'nprogress/nprogress.css';