Test Class Code coverage for a trigger is 70% and Error as :Only top-level class methods can be declared static

Can any one help on this .I have a trigger written on Case object for the duplication to get it closed .I have written a Test class where it gives the code coverage as 70% only. When i Execute in Developer Console it throws an Error as :

Line: 3, Column: 24
Only top-level class methods can be declared static

Trigger :

trigger Case_DuplicateClose on Case (before insert) {
    Map<String, Case> newCaseMap = new Map<String, Case>();
    for(Case c : trigger.new)
    {
        while(c.Subject.startsWith('RE:') || c.Subject.startsWith('Re:') || c.Subject.startsWith('FW:') || c.Subject.startsWith('Fw:'))
            c.Subject = c.Subject.substring(3).trim();
        if(newCaseMap.containsKey(c.Subject))
            c.Status = 'Closed';
        else
            newCaseMap.put(c.Subject, c);
    }
    for(Case c : [Select Subject From Case Where Subject in :newCaseMap.keySet()])
        newCaseMap.get(c.Subject).status = 'Closed';
}

@istest 
private class TestCase_DuplicateClose{ 
static testmethod void testcaseDuplicate(){ 
list <case>cases = new list<case>{};
for(integer i=0;i<200;i++){
case c=new case(Subject='RE:'+i);
cases.add(c);
}
test.startTest();
insert cases;
test.stopTest();
List<case> insertedcases =[Select Subject,status from case where id in:cases];
for(case c :insertedcases){
system.assertEquals(
'Closed',c.status);
}
}
}