Introduce new test functions to check logs

New functions log_contains, num_log_contains, num_log_has and num_log_has_re
are introduced in the conftest module to help and simplify checking:
- if logs contain a string;
- count how many messages contain a string;
- count how many messages are the given string;
- count how many messages matchs a regex.

A couple of existing tests are changed using the new functions.

tests/plugins/test_pairlist.py

@@ -15,7 +15,7 @@ from freqtrade.plugins.pairlist.pairlist_helpers import expand_pairlist
 from freqtrade.plugins.pairlistmanager import PairListManager
 from freqtrade.resolvers import PairListResolver
 from tests.conftest import (create_mock_trades, get_patched_exchange, get_patched_freqtradebot,
-                            log_has, log_has_re)
+                            log_has, log_has_re, num_log_has)
 
 
 @pytest.fixture(scope="function")
@@ -240,16 +240,14 @@ def test_remove_logs_for_pairs_already_in_blacklist(mocker, markets, static_pl_c
     new_whitelist = freqtrade.pairlists.verify_blacklist(whitelist + ['BLK/BTC'], logger.warning)
     # Ensure that the pair is removed from the white list, and properly logged.
     assert set(whitelist) == set(new_whitelist)
-    matches = sum(1 for message in caplog.messages
-                  if message == 'Pair BLK/BTC in your blacklist. Removing it from whitelist...')
-    assert matches == 1
+    assert num_log_has('Pair BLK/BTC in your blacklist. Removing it from whitelist...',
+                       caplog) == 1
 
     new_whitelist = freqtrade.pairlists.verify_blacklist(whitelist + ['BLK/BTC'], logger.warning)
     # Ensure that the pair is not logged anymore when being removed from the pair list.
     assert set(whitelist) == set(new_whitelist)
-    matches = sum(1 for message in caplog.messages
-                  if message == 'Pair BLK/BTC in your blacklist. Removing it from whitelist...')
-    assert matches == 1
+    assert num_log_has('Pair BLK/BTC in your blacklist. Removing it from whitelist...',
+                       caplog) == 1
 
 
 def test_refresh_pairlist_dynamic(mocker, shitcoinmarkets, tickers, whitelist_conf):