Fix three frontend specs for the Vue 3 compiler

What does this MR do and why?

These spec changes are needed before CI can run the Jest lanes with the Vue 3 template compiler. They also work with the Vue 2 compiler, so they can merge now. After that, the compiler switch MR contains only the switch.

• dap_impact_panels_spec.js: the spec expected a space between two table cells. The Vue 3 compiler removes whitespace between elements, so the row text has no space before "DAP Impact group". The regex no longer requires that space.
• workplan_spec.js: the spec clicked a dropdown item right after it opened the dropdown. With the Vue 3 compiler, the items are slot content of GlBaseDropdown, so opening the dropdown renders them again with new keys. The click landed on the old, unmounted item, and Vue 3 dropped its action event. The spec now waits for aria-expanded="true" on the toggle before it clicks.
• shared_sfc_mutation_spec.js: the spec tests the @vue/compat patch for an options object that the Vue 2 and the Vue 3 runtime share. Production compiles that object with the Vue 2 compiler until Vue 2 is removed. With the Vue 3 template compiler, Jest built the fixtures for Vue 3 only, so the Vue 2 runtime could not render them. The two template fixtures are now plain options objects that compile their template with compileTemplate from vue/compiler-sfc, the Vue 2 call that vue-loader 15 makes. So the spec runs in every lane. Skipping it instead would leave the patch untested, because no other job runs its Vue 3 tests.

How to verify

I ran the specs locally on this branch, with the fixtures from a master pipeline:

Spec	Default (Vue 2)	VUE_VERSION=3 VUE_COMPILER_VERSION=3
Both integration specs (yarn jest:integration)	33 passed	33 passed
shared_sfc_mutation_spec.js	2 passed, 7 skipped (Vue 3 only tests, as on master)	9 passed

With the master version of the two integration specs, the Vue 3 compiler lane fails 2 tests, one in each spec. That proves the change is necessary.

shared_sfc_mutation_spec.js also passes 9 of 9 with VUE_VERSION=3 and the Vue 2 compiler.

References

• Part of #634491
• Moved from !263251 (merged), which runs the Jest lanes with the Vue 3 compiler in CI
• Same plan as !263207, the Vue 2 removal POC
• Skip replaced after review in !263821 (merged), because it left the @vue/compat patch from 74eaa347 untested