There is no Compile option to run converage test for abseil

**Describe the bug**
- here in abseil-cpp repo, we unable to know the converege rate, but many of other repo have this
- this is import for us to know whether our source code has been tested fully or not

**Steps to reproduce the bug**
- check the compile file in cmake dir, there is no compile option like run converage `-ftest-converatge`

**What version of Abseil are you using?**
- here we hope this is an compile option for run converage test

**What operating system and version are you using**
ubuntu 18.04

**What compiler and version are you using?**
- current master branch

**What build system are you using?**

ubuntu 18.04

**Additional context**

Add any other context about the problem here.