Speed up backend tests in Gitpod environment

What does this MR do?

This MR speeds up backend tests in Gitpod environment.

By setting the environment variable GITLAB_TEST_EAGER_LOAD=false disable Rails' eager loading of class in the test environment which improves the load time of specs.

This way Gitpod users don't need to set Variables anymore.

Screenshots or Screencasts (strongly suggested)

To simulate the behaviour we are setting GITLAB_TEST_EAGER_LOAD to true and false explicitly.

Before (GITLAB_TEST_EAGER_LOAD=true)	After (GITLAB_TEST_EAGER_LOAD=false)	After via Gitpod

How to setup and validate locally (strongly suggested)

1. 🖱 Click Open in Gitpod below
2. ⏳ ⏳ ⏳ Wait for Gitpod to boot
3. ⌨ In Gitpod Terminal
   1. Verify that envvar is set to false via echo $GITLAB_TEST_EAGER_LOAD
   2. Verify speedup via time bin/rspec spec/models/ability_spec.rb
   3. Run this command at least twice to get relevant timings

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • Although it's a ~performance improvement for Gitpod users when running backend specs I am unsure if we need a changelog entry.
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team