test(ssr-compiler): specific fixture for lowercased attribute names

[ekashida]

Addresses a deviation in behavior for SSR.

[nolanlawson]

I think we should instead try to make engine-server emit attributes as lowercase when in the HTML namespace. I.e. fix this:

lwc/packages/@lwc/engine-server/src/__tests__/fixtures/attribute-modify-uppercase/expected.html

Line 1 in 906c53e

<x-cmp ARIA-LABEL="haha" DATA-FOO="bar" data-lwc-host-mutated="aria-label data-bar data-foo">

HTML is case-insensitive, so either way is fine. But under the hood, we need to think about deduping anyway:

connectedCallback() {
  this.setAttribute('FOO-BAR', 'a')
  this.removeAttribute('foo-bar') // should remove the attribute
}

So to solve this, we may as well lowercase the attribute when this.setAttribute is called.

For the SVG/MathML namespace we actually do have to care about case-sensitivity (e.g. SVG viewBox) so preserving the case is something we're going to have to tackle elsewhere. But it doesn't apply to the this.*Attribute case since LWC component hosts are only in the HTML namespace.