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>

command/builder_test.go

@@ -7,6 +7,7 @@ import (
 )
 
 func TestBuild(t *testing.T) {
+	t.Parallel()
 	c := command.New(nil, nil, "test", nil)
 	stubHandler := func([]string) error { panic("unreachable") }