There is no Compile option to run converage test for abseil

[littleDrew]

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.

[littleDrew]

• raise pr for this https://github.com/abseil/abseil-cpp/pull/1078