don't trigger default warning handler for inline servers

we already handle server warnings in StashWarnings middleware, so it's not
necessary to do so. without this, under TEST_VERBOSE=1 mode, those warnings
will be bubbled up to Test::NoWarnings handler, which could cause
Test::NoWarnings::had_no_warnings() to fail.
Bump the Plack::Middleware::Test::StashWarnings dep for "verbose" option

As this is in developer dependencies, there is no harm to doing so in a
stable series.
Merge branch '4.0/stash-warnings-verbose-option' into 4.0-trunk