treewide: parallel tests

Most tests already had no global state, however parallel was never enabled. This change enables it for all applicable tests.

Signed-off-by: Ophestra <cat@gensokyo.uk>

hst/fsephemeral_test.go

@@ -9,6 +9,8 @@ import (
 )
 
 func TestFSEphemeral(t *testing.T) {
+	t.Parallel()
+
 	checkFs(t, []fsTestCase{
 		{"nil", (*hst.FSEphemeral)(nil), false, nil, nil, nil, "<invalid>"},