Implemented shifter and its test bench code fully

tests/tests.sv

@@ -1,4 +1,5 @@
 `include "tests/seven_seg_tb.sv"
 `include "tests/lu_tb.sv"
 `include "tests/asu_tb.sv"
-`include "tests/instruction_classifier_tb.sv"
+`include "tests/instruction_classifier_tb.sv"
+`include "tests/shifter_tb.sv"