tests: fixed a random bug on test_grab_with_tuple_percents() (fixes BoboTiG#142)

Author: BoboTiG

CHANGELOG

@@ -6,6 +6,7 @@ History:
       - removed usage of deprecated "license_file" option for "license_files"
       - fixed flake8 usage in pre-commit
       - MSS: the implementation is now thread-safe on all OSes (fixes #169)
+      - tests: fixed a random bug on test_grab_with_tuple_percents() (fixes #142)
       - :heart: contributors: @
 
 5.1.0   2020/04/30

setup.cfg

@@ -55,3 +55,6 @@ addopts =
     --failed-first
     -r fE
     -v
+    # Trait all tests as flaky by default
+    --force-flaky
+    --no-success-flaky-report

tox.ini

@@ -10,6 +10,7 @@ skip_missing_interpreters = True
 passenv = DISPLAY
 alwayscopy = True
 deps =
+    flaky
     pytest
     # Must pin that version to support PyPy3
     pypy3: numpy==1.15.4