Bug 1547481 [wpt PR 16533] - Add submissions/ to .gitignore, a=testonly

Automatic update from web-platform-tests
Add submissions/ to .gitignore

https://w3c-test.org/submissions/ is served from a subdirectory of the
wpt clone on the w3c-test.org and at any given time that subdirectory
holds about 25GB+ of clones of wpt PR branches.

Without this change, the manifest generator apparently now tries to walk
all 25GB+ of test files in the clones in that submissions/ directory.

--
Make the linter allow "w3c-test.org" in .gitignore

--

wpt-commits: 92532f1be08b3bbd7cabc63c89c68483b7cc7546, 8228d24aff265dbb9b35f78358ca07f9f869e207
wpt-pr: 16533

testing/web-platform/tests/.gitignore

@@ -47,3 +47,6 @@ scratch
 /css/dist_last
 /css/tools/cache
 /webaudio/idl/*
+
+# w3c-test.org PR-branch mirroring
+submissions/

testing/web-platform/tests/lint.whitelist

@@ -64,6 +64,9 @@ TRAILING WHITESPACE, INDENT TABS, CR AT EOL: *.sxg
 
 TRAILING WHITESPACE: xhr/resources/headers-some-are-empty.asis
 
+## .gitignore
+W3C-TEST.ORG: .gitignore
+
 ## Documentation ##
 
 W3C-TEST.ORG: README.md