Add ADR for test markers

docs/adr/006-test-markers.md

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+# Test markers to separate testing from styling
+
+Date: 2024-12-03
+
+Deciders: Abban Dunne, Corinna Hillebrand, Gabriel Birke, Tanuja D.
+
+## Status
+
+Accepted
+
+## Context
+
+Our unit and integration tests use class names for finding elements (to
+check their existence or to trigger their events). This leads to two
+problems:
+
+1. In cases where we want to improve the semantics of names for styling by
+   simplifying/restructuring the markup, we need to keep the old class
+   names to avoid breaking the tests. See Mobile-DE-11.
+2. In cases where we introduce a new element for similar purposes (e.g.
+   close button with and without text) we need to use the old class name
+   to avoid breaking the tests and need to specify the CSS with a
+   qualifying 2nd class.
+
+To avoid these problems, we would like to separate *style names* from
+*markers for testing*.
+
+
+## Decison
+
+- TBD: copy one of the options considered
+- TBD: When do we do this (during the campaign, not until after the campaign)
+
+
+### Other options considered
+
+#### Test markers as unique IDs in data attributes
+
+Example
+
+```html
+<button class="wmde-banner-close-button" data-tm="soft-close-close-button">
+  <Icon/>
+</button>
+```
+
+```typescript
+await wrapper.find( '[data=soft-close-close-button]' ).trigger( 'click' );
+```
+
+### Test markers with local semantic meaning in data attributes
+
+Example
+
+```html
+<div class="wmde-banner-soft-close" data-tm="soft-close">
+    <button class="wmde-banner-close-button" data-tm="close-button">
+      <Icon/>
+    </button>
+    <!-- Other elements go here !>
+</div>
+```
+
+```typescript
+await wrapper.find( '[data=soft-close] [data=close-button]' ).trigger( 'click' );
+```
+
+Benefits:
+
+- Shorter, composable marker names
+- Allows for reusable components, without having to make their test marker name a
+  property
+- Feels "expressive"
+- Less probability of name collision when introducing a new element with
+new meaning (context), but same name as existing 
+
+
+Drawbacks:
+
+- Puts more burden on the developer to find the right "path" of selectors
+  in the tests
+
+### Test markers in class names
+
+```html
+<button class="wmde-banner-close-button tm-soft-close-close-button">
+  <Icon/>
+</button>
+```
+
+```typescript
+await wrapper.find( 'tm-soft-close-close-button]' ).trigger( 'click' );
+```
+
+Benefits:
+- More performant selectors (See [Don't use HTML data attributes to find
+elements with JS][1])
+- More compact banner code
+
+
+Drawbacks:
+- No enforcement to using only the test marker class names in tests
+- "Overloading" the class attribute, only implicit separation of concerns
+
+### Doing nothing, keep the status quo
+
+We consider the problems outlined in the "Context" section edge cases that
+don't necessitate the additional effort. We'll revisit this decision and
+this document after the 2024 and 2025 campaign in January 2024 and 2025.
+We'll check of many of these edge cases we encountered and if the
+additional effort and banner size of separating the concerns is worth it.
+
+
+## Consequences (in case we decide to do something)
+
+- Retrofitting old banners will take some work. We will do it feature by
+  feature, whenever we touch a test or develop a new one
+- "Naming things is one of the hardest problems in computer science". With
+  markers we're requiring more names.
+
+[1]: https://intu.io/blog/dont-use-data-attributes-to-find-html-elements-with-js/
+