Bug 1982404: split mochitests into their own kind r=taskgraph-reviewers,ahal

This patch moves mochitests out of the `test` kind and into their own kind. Aside from changing the `kind` and `source` metadata in tasks, it has no effect on task definitions.

On its own, this has no advantages. After we pick up [multiprocess kind evaluation](taskcluster/taskgraph#738) from upstream taskgraph this will make a notable improvement to `./mach taskgraph` runtime.

Differential Revision: https://phabricator.services.mozilla.com/D260707

UltraBlame original commit: 664d33826bb46965770be6166ade11b6e4c43d94

Author: marco-c

taskcluster/docs/kinds.rst

@@ -182,6 +182,17 @@ spidermonkey
 Spidermonkey tasks check out the full gecko source tree, then compile only the
 spidermonkey portion.  Each task runs specific tests after the build.
 
+mochitest
+---------
+
+See the :doc:`test kind documentation <kinds/test>` for more info.
+
+.. toctree::
+   :hidden:
+
+   kinds/test
+
+
 test
 ----
 

taskcluster/gecko_taskgraph/loader/test.py

@@ -119,7 +119,9 @@ def get_test_platforms(
     return test_platforms
 
 
-PREFIX_BY_KIND = {}
+PREFIX_BY_KIND = {
+    "mochitest": {"mochitest"},
+}
 
 
 @memoize