Does anyone know how to extend timeout value for mochitest?

[ISHIKAWA,chiaki]

I filed the following bugzilla.:
https://bugzilla.mozilla.org/show_bug.cgi?id=1636945
make timeout of mochitest much longer for running mochitest under
valgrind successfully.

valgrind slows down the execution of programs and thus the timeout value
is not optimal for running mochitest under valgrind.
(I am running a wrapper that invokes the original thunderbird binary
under valgrind masquerading as |thunderbird| binary.
If |make valgrind-test| is invoked, it may automatically extends the
timeout value by a factor.
But even then, we need to manually override the timeout value since the
slowdown factor is very much CPU-dependent.

So I want to know how to extend the timeout value for mochitest in general.

TIA

Chiaki