eval_started event: change interface to traceID\tjobsetID

I was not going to break the interface until I noticed
the current implementation uses the string literal \t.

t/Hydra/Event/EvalStarted.t

@@ -0,0 +1,94 @@
+use strict;
+use warnings;
+use Setup;
+use Hydra::Event;
+use Hydra::Event::EvalStarted;
+use Test2::V0;
+use Test2::Tools::Exception;
+use Test2::Tools::Mock qw(mock_obj);
+
+my $ctx = test_context();
+
+my $builds = $ctx->makeAndEvaluateJobset(
+    expression => "basic.nix",
+    build => 1
+);
+
+subtest "Parsing eval_started" => sub {
+    like(
+        dies { Hydra::Event::parse_payload("eval_started", "") },
+        qr/two arguments/,
+        "empty payload"
+    );
+    like(
+        dies { Hydra::Event::parse_payload("eval_started", "abc123") },
+        qr/two arguments/,
+        "one argument"
+    );
+    like(
+        dies { Hydra::Event::parse_payload("eval_started", "abc123\tabc123\tabc123") },
+        qr/two arguments/,
+        "three arguments"
+    );
+    like(
+        dies { Hydra::Event::parse_payload("eval_started", "abc123\tabc123") },
+        qr/should be an integer/,
+        "not an integer: second argument"
+    );
+    is(
+        Hydra::Event::parse_payload("eval_started", "abc123\t456"),
+        Hydra::Event::EvalStarted->new("abc123", 456)
+    );
+};
+
+subtest "interested" => sub {
+    my $event = Hydra::Event::EvalStarted->new(123, []);
+
+    subtest "A plugin which does not implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => ();
+
+        is($event->interestedIn($plugin), 0, "The plugin is not interesting.");
+    };
+
+    subtest "A plugin which does implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => (
+            add => [
+                "evalStarted" => sub {}
+            ]
+        );
+
+        is($event->interestedIn($plugin), 1, "The plugin is interesting.");
+    };
+};
+
+subtest "load" => sub {
+    my $jobset = $builds->{"empty_dir"}->jobset;
+
+    my $event = Hydra::Event::EvalStarted->new("traceID", $jobset->id);
+
+    $event->load($ctx->db());
+    is($event->{"jobset"}->get_column("id"), $jobset->id, "The jobset record matches.");
+
+    # Create a fake "plugin" with a evalStarted sub, the sub sets this
+    # "global" passedTraceID, passedJobset
+    my $passedTraceID;
+    my $passedJobset;
+    my $plugin = {};
+    my $mock = mock_obj $plugin => (
+        add => [
+            "evalStarted" => sub {
+                my ($self, $traceID, $jobset) = @_;
+                $passedTraceID = $traceID;
+                $passedJobset = $jobset;
+            }
+        ]
+    );
+
+    $event->execute($ctx->db(), $plugin);
+    is($passedTraceID, "traceID", "The plugin is told what the trace ID was");
+    is($passedJobset->get_column("id"), $jobset->id, "The plugin's evalStarted hook is called with the right jobset");
+};
+
+done_testing;