Bug 1587463 - Bump mozrunner version for web-platform tests. r=jgraham

Differential Revision: https://phabricator.services.mozilla.com/D49781

--HG--
extra : moz-landing-system : lando

testing/web-platform/tests/tools/wptrunner/requirements_firefox.txt

@@ -6,6 +6,6 @@ mozleak==0.2
 moznetwork==0.27
 mozprocess==1.0.0
 mozprofile==2.3.0
-mozrunner==7.5.1
+mozrunner==7.6.0
 mozversion==2.1.0
 psutil==5.6.3