Refactor t/session_in_template.t to Plack::Test

Move the test from LWP::UserAgent and Test::TCP to Plack::Test
Fix indentation
Rewrite session cycle test:

Another removal of LWP::UserAgent and Test::TCP, but also a fix for
something that has been nagging us for a bit.

When you ran the session cycle test, you would litter the t/sessions/
directory with session files that wouldn't get cleaned up.
We really don't need this test anymore:

In order to use LWP::Protocol::PSGI, just do:

    use MyApp;
    use LWP::UserAgent;
    use LWP::Protocol::PSGI;

    my $app = MyApp->to_app;

    LWP::Protocol::PSGI->register($app);
    my $ua = LWP::UserAgent->new();

    ...
another one
another one
another one
last one
remove unused modules
last one
replace last usage of LWP::UserAgent with HTTP::Tiny
reflect changes
Merge branch 'pr/move_to_plack_test'