Feature Management Class

To check Custom Permission and Other Features

Check and modify the values of feature parameters, and to show or hide custom objects and custom permissions in your subscribers’ orgs.

Syntax:

FeatureManagement.checkPermission('CustomPermission_apiName');

Return Type:Boolean

Example : 

public static Boolean havingCperm = FeatureManagement.checkPermission(apiName);

OR

if(FeatureManagement.checkPermission(apiName)){your logic}

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For information about feature parameters, see “Get Started with the Feature Management App” in the ISVforce Guide.

In TestClass  to assign PermissionSet Use: 

PermissionSet ps = [SELECT Id,Name FROM PermissionSet WHERE Name = 'apiName'];

PermissionSetAssignment psa = new PermissionSetAssignment();
psa.AssigneeId = UserInfo.getUserId();
psa.PermissionSetId = ps.Id;
insert psa;