Unset GIT_ASKPASS in tests that assume it is unset

[EliahKagan]

Two of the askpass tests assume GIT_ASKPASS is not set. Before this change, they wrongly fail when it is set, even if the code under test is working correctly.

1. ssh_askpass_is_used_as_fallback
2. ssh_askpass_does_not_override_current_value

As other tests that deliberately involve GIT_ASKPASS (and therefore do not need to be modified) attest, when it is set:

1. SSH_ASKPASS is not used as a fallback (GIT_ASKPASS deliberately takes precedence).

2. Although SSH_ASKPASS still does not override the current value, GIT_ASKPASS does override it, so checking for equality to the current value fails.

Since those test cases are among those that already have the serial attribute and already temporarily modify the environment, this extends that modification by having it also temporarily unset GIT_ASKPASS if it was set.

An alternative fix could be to pass false as the first argument to apply_environment, so that GIT_ASKPASS is not considered. Which of these approaches is better may depend on exactly what behavior we are trying to test. (Maybe even two more test cases should be added, to test it both ways.)

---

Situations where GIT_ASKPASS is set include dev containers. I discovered this bug while running tests in the dev container environment added in #1502, where all tests passed except the two mentioned above:

failures:

---- options::apply_environment::ssh_askpass_does_not_override_current_value stdout ----
thread 'options::apply_environment::ssh_askpass_does_not_override_current_value' panicked at gix-prompt/tests/options/mod.rs:68:9:
assertion `left == right` failed
  left: "/vscode/bin/linux-x64/eaa41d57266683296de7d118f574d0c2652e1fc4/extensions/git/dist/askpass.sh"
 right: "current"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- options::apply_environment::ssh_askpass_is_used_as_fallback stdout ----
thread 'options::apply_environment::ssh_askpass_is_used_as_fallback' panicked at gix-prompt/tests/options/mod.rs:50:9:
assertion `left == right` failed
  left: "/vscode/bin/linux-x64/eaa41d57266683296de7d118f574d0c2652e1fc4/extensions/git/dist/askpass.sh"
 right: "fallback"


failures:
    options::apply_environment::ssh_askpass_does_not_override_current_value
    options::apply_environment::ssh_askpass_is_used_as_fallback

test result: FAILED. 7 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 1.70s

error: test failed, to rerun pass `-p gix-prompt --test prompt`

All other tests passed already, and all tests pass with this change.

[Byron]

Thanks a lot!

(I merged in a rush, if I missed anything in the PR description please let me know)

[EliahKagan]

(I merged in a rush, if I missed anything in the PR description please let me know)

You probably did not miss anything. But if you have thoughts on this then I am interested and I'd be pleased to make a follow-up PR if called for:

An alternative fix could be to pass false as the first argument to apply_environment, so that GIT_ASKPASS is not considered. Which of these approaches is better may depend on exactly what behavior we are trying to test.

(But if you saw that and didn't have a response at this time, that is also definitely fine.)