You appear to have this relationship:
FInal_Results::ID = TestCase::ID
Which is a one to many relationship from Final_Results to testCase. (for every record in Final_Results, there could be many related records in TestCase.)
I want to display the related testcases in a portal
And this portal will be placed on a layout based on what table? Final_Results?
along with a dropdown filed for the results(Which is not the part of the testcase table)
Presumably that dropdown field is the results field you list in Test_Results. But then why would this drop down field be defined in Test_Results, but placed in a portal to TestCases? If I have the relationships and layout design correct, that field would show the same value in every row of your portal, you could not record a different value for each related testcase record.
Note: I also tried analyzing this from the idea that you have one TestCase record related to many Test_Results record and that has even more problems than what I have pointed out here for the opposite type of relationship.