Run SHLVL tests sequentially (#14727)

Tests for #14707 are causing hangs which are preventing `toolkit test`
from completing on some systems. This appears to be due to the use of
`-i` to force interactive mode, which is required in order to update the
`SHLVL`. Both tests are likely attempting to acquire the terminal at the
same time, and one is hanging as a result.

This is a temporary fix which runs both of these tests sequentially.
It's temporary because we need to find a solution which doesn't use
`-i`, since any other future `-it` test will cause the same situation
again.

tests/shell/environment/env.rs

@@ -238,10 +238,7 @@ fn env_shlvl() {
     ");
 
     assert_eq!(actual.out, "6");
-}
 
-#[test]
-fn env_shlvl_in_exec() {
     let actual = nu!("
         $env.SHLVL = 29
         nu -i -c \"exec nu -i -c 'print $env.SHLVL'\"