Fix with gradle plugin config

[petertrr]

Gradle file API returns absolute paths relative to project's directory, so no need to check whether it's absolute. Need to use proper way to specify path in build.gradle instead.
Added more tests.

What's done:

• Fixes & tests

This pull request closes #703

[codecov]

Codecov Report

Merging #706 (00e6bda) into master (b43fb71) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #706      +/-   ##
============================================
+ Coverage     80.10%   80.13%   +0.02%     
- Complexity     1894     1895       +1     
============================================
  Files            90       90              
  Lines          4912     4912              
  Branches       1575     1575              
============================================
+ Hits           3935     3936       +1     
+ Misses          239      238       -1     
  Partials        738      738              

Flag	Coverage Δ	Complexity Δ
unittests	80.13% <100.00%> (+0.02%)	0.00 <3.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...n/org/cqfn/diktat/plugin/gradle/DiktatExtension.kt	100.00% <100.00%> (ø)	7.00 <1.00> (+1.00)
...rg/cqfn/diktat/plugin/gradle/DiktatGradlePlugin.kt	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
...qfn/diktat/plugin/gradle/DiktatJavaExecTaskBase.kt	92.98% <100.00%> (+1.75%)	17.00 <2.00> (ø)

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[aktsay6]

lgtm