Workaround SimpleCov / Coverage limitation around `load`

What does this MR do and why?

See #408630

Before:

(arm64) tkgl2:gitlab tkuah$ rm -rf coverage/
(arm64) tkgl2:gitlab tkuah$ bundle exec rspec --seed 37811 -f d /Users/tkuah/code/gdk-ee/gitlab/spec/initializers/active_record_transaction_observer_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Randomized with seed 37811

Test environment set up in 5.671124 seconds

ActiveRecord Transaction Observer
  # order random
  when DBMS is available
    calls Gitlab::Database::Transaction::Observer
    when flipper table does not exist
      does not calls Gitlab::Database::Transaction::Observer
  when DBMS is not available
    does not calls Gitlab::Database::Transaction::Observer

Finished in 8.05 seconds (files took 12.26 seconds to load)
3 examples, 0 failures

Randomized with seed 37811

[TEST PROF INFO] Time spent in factories: 00:00.006 (0.06% of total time)
Coverage report generated for RSpec to /Users/tkuah/code/gdk-ee/gitlab/coverage. 64827 / 380515 LOC (17.04%) covered.
Coverage report generated for RSpec to /Users/tkuah/code/gdk-ee/gitlab/coverage/coverage.xml
Lcov style coverage report generated for RSpec to /Users/tkuah/code/gdk-ee/gitlab/coverage/lcov/gitlab.lcov
(arm64) tkgl2:gitlab tkuah$ grep -A10 transaction_observer.rb /Users/tkuah/code/gdk-ee/gitlab/coverage/coverage.xml 
        <class name="active_record_transaction_observer" filename="config/initializers/active_record_transaction_observer.rb" line-rate="0.75" branch-rate="0" complexity="0">
          <methods/>
          <lines>
            <line number="3" branch="false" hits="1"/>
            <line number="5" branch="false" hits="1"/>
            <line number="8" branch="false" hits="1"/>
            <line number="10" branch="false" hits="1"/>
            <line number="14" branch="false" hits="1"/>
            <line number="16" branch="false" hits="0"/>
            <line number="19" branch="false" hits="1"/>
            <line number="20" branch="false" hits="0"/>

After :

(arm64) tkgl2:gitlab tkuah$ rm -rf coverage/
(arm64) tkgl2:gitlab tkuah$ bundle exec rspec --seed 37811 -f d /Users/tkuah/code/gdk-ee/gitlab/spec/initializers/active_record_transaction_observer_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Randomized with seed 37811

Test environment set up in 5.780364 seconds

ActiveRecord Transaction Observer
  # order random
  when DBMS is available
    calls Gitlab::Database::Transaction::Observer
    when flipper table does not exist
      does not calls Gitlab::Database::Transaction::Observer
  when DBMS is not available
    does not calls Gitlab::Database::Transaction::Observer

Finished in 14.89 seconds (files took 12.34 seconds to load)
3 examples, 0 failures

Randomized with seed 37811

[TEST PROF INFO] Time spent in factories: 00:00.005 (0.03% of total time)
Coverage report generated for RSpec, RSpec1, RSpec11, RSpec111 to /Users/tkuah/code/gdk-ee/gitlab/coverage. 64798 / 380514 LOC (17.03%) covered.
Coverage report generated for RSpec, RSpec1, RSpec11, RSpec111 to /Users/tkuah/code/gdk-ee/gitlab/coverage/coverage.xml
Lcov style coverage report generated for RSpec, RSpec1, RSpec11, RSpec111 to /Users/tkuah/code/gdk-ee/gitlab/coverage/lcov/gitlab.lcov
(arm64) tkgl2:gitlab tkuah$ grep -A10 transaction_observer.rb /Users/tkuah/code/gdk-ee/gitlab/coverage/coverage.xml 
        <class name="active_record_transaction_observer" filename="config/initializers/active_record_transaction_observer.rb" line-rate="1.0" branch-rate="0" complexity="0">
          <methods/>
          <lines>
            <line number="3" branch="false" hits="4"/>
            <line number="5" branch="false" hits="4"/>
            <line number="8" branch="false" hits="4"/>
            <line number="10" branch="false" hits="1"/>
            <line number="14" branch="false" hits="4"/>
            <line number="16" branch="false" hits="1"/>
            <line number="19" branch="false" hits="4"/>
            <line number="20" branch="false" hits="1"/>

Workaround SimpleCov / Coverage limitation around load

SimpleCov only reports on coverage for the last Kernel.load call, so we are missing coverage data when we test initializers. Add a workaround to spin up new SimpleCov to cover, and merge each load.

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