Add tests for check functions

Relies on capturing stdout.

See: https://docs.pytest.org/en/5.0.1/capture.html

tests/test_check.py

@@ -0,0 +1,64 @@
+import csv_metadata_quality.check as check
+import pytest
+
+def test_check_invalid_issn(capsys):
+    '''Test checking invalid ISSN.'''
+
+    value = '2321-2302'
+
+    check.issn(value)
+    
+    captured = capsys.readouterr()
+    assert captured.out == f'Invalid ISSN: {value}\n'
+
+
+def test_check_valid_issn():
+    '''Test checking valid ISSN.'''
+
+    value = '0024-9319'
+
+    result = check.issn(value)
+
+    assert result == value
+
+
+def test_check_invalid_isbn(capsys):
+    '''Test checking invalid ISBN.'''
+
+    value = '99921-58-10-6'
+
+    check.isbn(value)
+
+    captured = capsys.readouterr()
+    assert captured.out == f'Invalid ISBN: {value}\n'
+
+
+def test_check_valid_isbn():
+    '''Test checking valid ISBN.'''
+
+    value = '99921-58-10-7'
+
+    result = check.isbn(value)
+
+    assert result == value
+
+
+def test_check_invalid_separators(capsys):
+    '''Test checking invalid multi-value separators.'''
+
+    value = 'Alan|Orth'
+
+    check.separators(value)
+
+    captured = capsys.readouterr()
+    assert captured.out == f'Invalid multi-value separator: {value}\n'
+
+
+def test_check_valid_separators():
+    '''Test checking valid multi-value separators.'''
+
+    value = 'Alan||Orth'
+
+    result = check.separators(value)
+
+    assert result == value