Fix test-http2-session-timeout.js and move to parallel

First commit:

test: make http2 timeout test robust

Instead of using magic values for the server timeout in
test-http2-session-timeout, measure the duration of the first request
(which should be longer than subsequent requests) and use that value.

Fixes: https://github.com/nodejs/node/issues/20628

Second commit:

test: move http2 test to parallel

A fix to test-http2-session-timeout makes it sufficiently robust that it
can be moved to the parallel directory.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines