Fix a bug in AbsCpatureTimeExtension

[enobufs]

Description

• Small modifications to the following method to handle a negative captureClockOffset value correctly.
  • EstimatedCaptureClockOffsetDuration
  • NewAbsCaptureTimeExtensionWithCaptureClockOffset
• Added a unit test to verify the fix

Reference issue

Fixes #247

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.50%. Comparing base (78da5a2) to head (af67ac0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #248 +/- ## ========================================== + Coverage 87.41% 87.50% +0.09% ========================================== Files 21 21 Lines 2113 2129 +16 ========================================== + Hits 1847 1863 +16 Misses 230 230 Partials 36 36 ``` | [Flag](https://app.codecov.io/gh/pion/rtp/pull/248/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [go](https://app.codecov.io/gh/pion/rtp/pull/248/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `87.50% <100.00%> (+0.09%)` | :arrow_up: | | [wasm](https://app.codecov.io/gh/pion/rtp/pull/248/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `86.98% <100.00%> (+0.09%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#carryforward-flags-in-the-pull-request-comment) to find out more.

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