Improve test sensors to execute them only when the related property is defined

[Evangelink]

Update sensors so they are executed only when the related property is defined.

For example test sensors should be activated only when one of the following property is set (see here):
/d:sonar.cs.vstest.reportsPaths=
/d:sonar.cs.nunit.reportsPaths=
/d:sonar.cs.xunit.reportsPaths=

Same behavior for the code coverage sensor (see here):
/d:sonar.cs.vscoveragexml.reportsPaths=
/d:sonar.cs.dotcover.reportsPaths=
/d:sonar.cs.opencover.reportsPaths=
/d:sonar.cs.ncover3.reportsPaths=

[Evangelink]

This is currently impossible to solve this ticket because it is not possible to provide a list of possible arguments to look for to be executed. We need to wait for this ticket (SONAR-8546) to be fixed first.

[Evangelink]

Feature will be available on SonarQube 6.5