Supplying absolute path for cert files in tests

[srijeet0406]

This PR is not related to any issue. It makes sure that we are supplying absolute paths to the certs in our api tests.

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Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Create client certs
Make sure the tests pass.
Manual testing:
Follow the steps in #7392

If this is a bugfix, which Traffic Control versions contained the bug?

• master

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov]

Codecov Report

Merging #7742 (a59fed8) into master (a0f6b1f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #7742   +/-   ##
=========================================
  Coverage     32.32%   32.32%           
  Complexity       98       98           
=========================================
  Files           710      710           
  Lines         79815    79815           
  Branches        965      965           
=========================================
  Hits          25798    25798           
  Misses        51870    51870           
  Partials       2147     2147           

Flag	Coverage Δ
traffic_ops_integration	69.39% <ø> (ø)
v3	57.79% <ø> (ø)
v4	79.18% <ø> (ø)
v5	78.52% <ø> (ø)

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[zrhoffman]

Please add labels to #7742.