Add test for Plugin::ConfigStr()

[AzulRadio]

Signed-off-by: youhy haoyuan2019@outlook.com

New feature

Summary

Add test for Plugin::ConfigStr()

Test it

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #447 (773bfd4) into ign-gui3 (33bbf41) will increase coverage by 1.50%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-gui3     #447      +/-   ##
============================================
+ Coverage     67.81%   69.32%   +1.50%     
============================================
  Files            29       29              
  Lines          3281     3276       -5     
============================================
+ Hits           2225     2271      +46     
+ Misses         1056     1005      -51     

Impacted Files	Coverage Δ
src/Plugin.cc	82.68% <ø> (+21.24%)	⬆️

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[jennuine]

Let's go ahead and wrap this in LCOV_EXCL_START/STOP as well:

gz-gui/src/Plugin.cc

Lines 365 to 366 in e0acd4a

	ignwarn << "There was an error parsing the plugin element for " <<
	"[" << this->title << "]." << std::endl;

[jennuine]

@osrf-jenkins run tests please