Add easy ways to turn on event storage

[schwern]

#198 made Test::Builder stop storing events by default to conserve memory. Right now it's a PITA to turn event storage on.

use Test::Builder;
use TB2::History;

my $Test = Test::Builder->new;
my $history = TB2::History->new( store_events => 1 );
$Test->test_state->ec->history($history);

Make it convenient to turn them back on again both in Test::Builder::Module and Test::Builder.

use Test::More store_events => 1;

Test::Builder->new( store_events => 1 );

Keep in mind that event storage must be turned on before any tests or events (ie. the plan) is run.

[schwern]

c3ee922 adds the ability to turn storage on in import and via Test::Builder directly. That should do it.