Test using Hydra with flakes

It seemed there was no self-contained end-to-end test actually doing
this?!

Among other things, this will help ensure that the switch-over to
`nix-eval-jobs` is correct.

t/evaluator/evaluate-flake.t

@@ -0,0 +1,67 @@
+use feature 'unicode_strings';
+use strict;
+use warnings;
+use Setup;
+use Test2::V0;
+use File::Copy qw(cp);
+
+my $ctx = test_context(
+    nix_config => qq|
+    experimental-features = nix-command flakes
+    |,
+    hydra_config => q|
+    <runcommand>
+      evaluator_pure_eval = false
+    </runcommand>
+    |
+);
+
+sub checkFlake {
+    my ($flake) = @_;
+
+    cp($ctx->jobsdir . "/basic.nix", $ctx->jobsdir . "/" . $flake);
+    cp($ctx->jobsdir . "/config.nix", $ctx->jobsdir . "/" . $flake);
+    cp($ctx->jobsdir . "/empty-dir-builder.sh", $ctx->jobsdir . "/" . $flake);
+    cp($ctx->jobsdir . "/fail.sh", $ctx->jobsdir . "/" . $flake);
+    cp($ctx->jobsdir . "/succeed-with-failed.sh", $ctx->jobsdir . "/" . $flake);
+
+    chmod 0755, $ctx->jobsdir . "/" . $flake . "/empty-dir-builder.sh";
+    chmod 0755, $ctx->jobsdir . "/" . $flake . "/fail.sh";
+    chmod 0755, $ctx->jobsdir . "/" . $flake . "/succeed-with-failed.sh";
+
+    my $builds = $ctx->makeAndEvaluateJobset(
+        flake => 'path:' . $ctx->jobsdir . "/" . $flake,
+        build => 1
+    );
+
+    subtest "Build: succeed_with_failed" => sub {
+        my $build = $builds->{"succeed_with_failed"};
+
+        is($build->finished, 1, "Build should be finished.");
+        is($build->buildstatus, 6, "succeeeded-but-failed should have buildstatus 6.");
+    };
+
+    subtest "Build: empty_dir" => sub {
+        my $build = $builds->{"empty_dir"};
+
+        is($build->finished, 1, "Build should be finished.");
+        is($build->buildstatus, 0, "Should have succeeded.");
+    };
+
+    subtest "Build: fails" => sub {
+        my $build = $builds->{"fails"};
+
+        is($build->finished, 1, "Build should be finished.");
+        is($build->buildstatus, 1, "Should have failed.");
+    };
+}
+
+subtest "Flake using `checks`" => sub {
+    checkFlake 'flake-checks'
+};
+
+subtest "Flake using `hydraJobs`" => sub {
+    checkFlake 'flake-hydraJobs'
+};
+
+done_testing;