Only run expectrl gix-prompt tests where supported

Two important `gix-prompt` tests simulate interactivity with
`expectrl`, which (non-optionally) depends on `ptyprocess`. But the
only (families of) Unix-like operating systems that `ptyprocess`
currently supports are Linux, FreeBSD, and macOS.

The tests that used `expectrl` were previously targeted to all
"unix" systems. This narrows them to systems where `target_os` is
`linux`, `freebsd`, or `macos`, and conditions the `expectrl` dev
dependency on those targets.

This way, running the test suite no longer fails to build on other
Unix-like systems. (In particular, this fixes the failure on
OmniOS, an illumos system. But the problem isn't illumos-specific.)

Author: EliahKagan

gix-prompt/Cargo.toml

@@ -27,4 +27,6 @@ parking_lot = "0.12.1"
 [dev-dependencies]
 gix-testtools = { path = "../tests/tools" }
 serial_test = { version = "3.1.0", default-features = false }
+
+[target.'cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))'.dev-dependencies]
 expectrl = "0.7.0"

gix-prompt/tests/prompt.rs

@@ -34,7 +34,7 @@ mod ask {
     }
 
     #[test]
-    #[cfg(unix)]
+    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
     fn askpass_only() {
         let mut cmd = std::process::Command::new(env!("CARGO"));
         cmd.args(["build", "--example", "use-askpass", "--example", "askpass"]);
@@ -48,7 +48,7 @@ mod ask {
     }
 
     #[test]
-    #[cfg(unix)]
+    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
     fn username_password() {
         let mut cmd = std::process::Command::new(env!("CARGO"));
         cmd.args(["build", "--example", "credentials"]);
@@ -65,7 +65,7 @@ mod ask {
     }
 
     #[test]
-    #[cfg(not(unix))]
+    #[cfg(not(any(target_os = "linux", target_os = "freebsd", target_os = "macos")))]
     #[ignore]
     fn username_password_not_available() {}
 }