feat: clean code, combine scripts, add tests

tests/test_poly_a.py

-import pytest
-from poly_a import poly_a_generator, poly_a_addition_to_fasta_list
-
+"""Test poly_a.py script."""
 
+import numpy as np
+from sequence_extractor.poly_a import (
+    poly_a_generator, poly_a_addition_to_fasta_list
+)
 
 
 def test_poly_a_generator():
-    assert poly_a_generator(exon_string) == exon_and_polya
+    """Test poly_a_generator function."""
+    exon_string = 'ACGGCTGGCACCTTGTTTGGGGAAGGATTCCGTGCCTTTG'
+    poly_a_length = np.random.randint(100, 250)
+    test_string = poly_a_generator(exon_string, poly_a_length)
+
+    assert len(test_string) == sum([len(exon_string), poly_a_length])
 
 
 def test_poly_a_addition_to_fasta_list():
-    assert poly_a_addition_to_fasta_list(list_of_tuples) == manipulated_list_of_tuples
+    """Test poly_a_addition_to_fasta_list function."""
+    exon_list = [('>ENST00000673477', 'ACGGCTGGCACCTTGTTTGGGGAAG'),
+                 ('>ENST00000378391', 'AAATACTGACGGACGTGG')]
+    poly_a_length = np.random.randint(100, 250)
+    test_list = poly_a_addition_to_fasta_list(exon_list, poly_a_length)
+
+    assert len(test_list[0][1]) == sum([len(exon_list[0][1]), poly_a_length])
+    assert len(test_list[1][1]) == sum([len(exon_list[1][1]), poly_a_length])