Bug 1571986 - Add some info to try to find what is going on. r=me

MANUAL PUSH: because it starts happening with something that landed on inbound.

taskcluster/scripts/misc/wrench-windows-tests.sh

@@ -27,3 +27,6 @@ powershell.exe 'iex (Get-Content -Raw ci-scripts\set-screenresolution.ps1); Set-
 export CARGOFLAGS='--verbose --frozen'
 export FREETYPE_CMAKE_GENERATOR=Ninja
 cmd.exe /c 'ci-scripts\windows-tests.cmd'
+
+# Diagnostic for bug 1571986.
+tasklist -M