fix(cdk/testing): prevent duplicate subscription (#22460)
fix(cdk/testing): prevent duplicate subscription (#22460)

If the actual subscription provided by the environment comes
after the first batched function call, the default function is
installed and without this change it'll stay installed forever,
even if another handler is installed afterwards.
mmalerba/material2,angular/components,mmalerba/material2,mmalerba/material2,DevVersion/material2,angular/components,mmalerba/material2,angular/components,mmalerba/material2,DevVersion/material2,josephperrott/material2,josephperrott/material2,crisbeto/material2,crisbeto/material2,DevVersion/material2,josephperrott/material2,DevVersion/material2,angular/components,josephperrott/material2,josephperrott/material2,crisbeto/material2,crisbeto/material2,DevVersion/material2