Add actual tests to the test file

tests/test_exon_concatenation.py

 import pytest
-
 import exon_concatenation from exon_concatenation
+
+test_fasta_1 = "test_files/test_1.fa"
+test_fasta_2 = "test_files/test_2.fa"
+
+def test_exon_concatenation():
+    assert exon_concatenation(test_fasta_1) == expected_list_of_tuples
+    assert exon_concatenation(test_fasta_2) == expected