Give each test its own Nix directories

Otherwise we risk tripping over confusing statuses where a build is
"done" and "Cached", but we were expecting to run it.

tests/evaluate-basic.t

@@ -0,0 +1,32 @@
+use feature 'unicode_strings';
+use strict;
+use Cwd;
+use Setup;
+
+(my $datadir, my $pgsql) = test_init();
+
+require Hydra::Schema;
+require Hydra::Model::DB;
+
+use Test2::V0;
+
+my $db = Hydra::Model::DB->new;
+hydra_setup($db);
+
+my $project = $db->resultset('Projects')->create({name => "tests", displayname => "", owner => "root"});
+
+# Most basic test case, no parameters
+my $jobset = createBaseJobset("basic", "basic.nix");
+
+ok(evalSucceeds($jobset),               "Evaluating jobs/basic.nix should exit with return code 0");
+is(nrQueuedBuildsForJobset($jobset), 3, "Evaluating jobs/basic.nix should result in 3 builds");
+
+for my $build (queuedBuildsForJobset($jobset)) {
+    ok(runBuild($build), "Build '".$build->job."' from jobs/basic.nix should exit with code 0");
+    my $newbuild = $db->resultset('Builds')->find($build->id);
+    is($newbuild->finished, 1, "Build '".$build->job."' from jobs/basic.nix should be finished.");
+    my $expected = $build->job eq "fails" ? 1 : $build->job =~ /with_failed/ ? 6 : 0;
+    is($newbuild->buildstatus, $expected, "Build '".$build->job."' from jobs/basic.nix should have buildstatus $expected.");
+}
+
+done_testing;