prevent env-altering tests to affect shallow tests

These are sensitive to GIT_SHALLOW_FILE, which can mess up clones.

Author: Byron

gix/tests/repository/shallow.rs

@@ -1,6 +1,8 @@
 use crate::util::{hex_to_id, named_subrepo_opts};
+use serial_test::parallel;
 
 #[test]
+#[parallel]
 fn no() -> crate::Result {
     for name in ["base", "empty"] {
         let repo = named_subrepo_opts("make_shallow_repo.sh", name, crate::restricted())?;
@@ -22,6 +24,7 @@ fn no() -> crate::Result {
 }
 
 #[test]
+#[parallel]
 fn yes() -> crate::Result {
     for name in ["shallow.git", "shallow"] {
         let repo = named_subrepo_opts("make_shallow_repo.sh", name, crate::restricted())?;
@@ -38,8 +41,10 @@ mod traverse {
     use gix_traverse::commit::Sorting;
 
     use crate::util::{hex_to_id, named_subrepo_opts};
+    use serial_test::parallel;
 
     #[test]
+    #[parallel]
     fn boundary_is_detected_triggering_no_error() -> crate::Result {
         for name in ["shallow.git", "shallow"] {
             let repo = named_subrepo_opts("make_shallow_repo.sh", name, crate::restricted())?;
@@ -50,6 +55,7 @@ mod traverse {
     }
 
     #[test]
+    #[parallel]
     fn complex_graphs_can_be_iterated_despite_multiple_shallow_boundaries() -> crate::Result {
         let base = gix_path::realpath(gix_testtools::scripted_fixture_read_only("make_remote_repos.sh")?.join("base"))?;
         let shallow_base = gix_testtools::scripted_fixture_read_only_with_args(