Consider compiling frontend test fixtures within the `prepare` stage

We currently need to compile our frontend fixtures twice in our CI pipelines (within both the karma and jest jobs). It would save quite a few compute cycles if we could find a way to do this just once in a prior step and then load the fixtures within karma and jest.

Alternatively, we may want to consider merging the karma and jest CI jobs into a single gitlab-ce~3412464 test job.

/cc @winh

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The following discussion from gitlab-ce!30917 should be addressed:

• @winh started a discussion: (+2 comments)

  1. It looks like you only adjusted Jest and not Karma. Was that intended?
  2. What do you think of making this a separate job that Jest and Karma can depend on? (this has follow-up character and may not be relevant if we get to https://gitlab.com/gitlab-org/gitlab-ce/issues/49127 soon)