sign: Skip gpg tests if gpg is not installed

This adds a guard to the gpg signing tests which will skip the test if
`gpg` is not installed on the system.

This is done in order to avoid requiring all collaborators to have setup
all the tools on their local machines that are required to test commit
signing.

lib/tests/test_gpg.rs

@@ -3,7 +3,7 @@ use std::fs::Permissions;
 use std::io::Write;
 #[cfg(unix)]
 use std::os::unix::prelude::PermissionsExt;
-use std::process::Stdio;
+use std::process::{Command, Stdio};
 
 use assert_matches::assert_matches;
 use insta::assert_debug_snapshot;
@@ -75,6 +75,15 @@ impl GpgEnvironment {
     }
 }
 
+macro_rules! gpg_guard {
+    () => {
+        if Command::new("gpg").arg("--version").status().is_err() {
+            eprintln!("Skipping test because gpg is not installed on the system");
+            return;
+        }
+    };
+}
+
 fn backend(env: &GpgEnvironment) -> GpgBackend {
     // don't really need faked time for current tests,
     // but probably will need it for end-to-end cli tests
@@ -87,6 +96,8 @@ fn backend(env: &GpgEnvironment) -> GpgBackend {
 
 #[test]
 fn gpg_signing_roundtrip() {
+    gpg_guard!();
+
     let env = GpgEnvironment::new().unwrap();
     let backend = backend(&env);
     let data = b"hello world";
@@ -111,6 +122,8 @@ fn gpg_signing_roundtrip() {
 
 #[test]
 fn gpg_signing_roundtrip_explicit_key() {
+    gpg_guard!();
+
     let env = GpgEnvironment::new().unwrap();
     let backend = backend(&env);
     let data = b"hello world";
@@ -142,6 +155,8 @@ fn gpg_signing_roundtrip_explicit_key() {
 
 #[test]
 fn unknown_key() {
+    gpg_guard!();
+
     let env = GpgEnvironment::new().unwrap();
     let backend = backend(&env);
     let signature = br"-----BEGIN PGP SIGNATURE-----
@@ -173,6 +188,8 @@ fn unknown_key() {
 
 #[test]
 fn invalid_signature() {
+    gpg_guard!();
+
     let env = GpgEnvironment::new().unwrap();
     let backend = backend(&env);
     let signature = br"-----BEGIN PGP SIGNATURE-----