Bug 1527156 [wpt PR 15331] - Update mozrunner to 7.3.0, a=testonly

Automatic update from web-platform-tests
Update mozrunner from 7.2.0 to 7.3.0
--

wpt-commits: e06d72647e3c51c43a954c4e8171b49149e9e567
wpt-pr: 15331

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

@@ -2,7 +2,7 @@ marionette_driver==2.7.0
 mozprofile==2.1.0
 mozprocess==1.0.0
 mozcrash == 1.0
-mozrunner==7.2.0
+mozrunner==7.3.0
 mozleak==0.2
 mozinstall==2.0.0
 mozdownload==1.25