Make EXE_INFO testable and add a basic test for it

This extracts the code used to lazily initialize `EXE_INFO` into a
separate `exe_info` helper, which is an implementation detail, but
can be called any number of times without caching from the tests.

It also adds one test.

The tests of downstream functions (those that use `EXE_INFO`)
remain more important. But this will allow the test to be expanded,
or another added, to test more expectations that `EXE_INFO` is
supposed to satisfy.

gix-path/src/env/git/mod.rs

@@ -80,7 +80,9 @@ pub(super) static EXE_NAME: &str = "git";
 /// Invoke the git executable to obtain the origin configuration, which is cached and returned.
 ///
 /// The git executable is the one found in PATH or an alternative location.
-pub(super) static EXE_INFO: Lazy<Option<BString>> = Lazy::new(|| {
+pub(super) static EXE_INFO: Lazy<Option<BString>> = Lazy::new(exe_info);
+
+fn exe_info() -> Option<BString> {
     let mut cmd = git_cmd(EXE_NAME.into());
     gix_trace::debug!(cmd = ?cmd, "invoking git for installation config path");
     let cmd_output = match cmd.output() {
@@ -98,7 +100,7 @@ pub(super) static EXE_INFO: Lazy<Option<BString>> = Lazy::new(|| {
     };
 
     first_file_from_config_with_origin(cmd_output.as_slice().into()).map(ToOwned::to_owned)
-});
+}
 
 fn git_cmd(executable: PathBuf) -> Command {
     let mut cmd = Command::new(executable);

gix-path/src/env/git/tests.rs

@@ -357,6 +357,19 @@ mod locations {
 
 use std::path::Path;
 
+#[test]
+fn exe_info() {
+    let path = super::exe_info()
+        .map(crate::from_bstring)
+        .expect("Nonempty config in the test environment");
+
+    assert!(
+        path.is_absolute(),
+        "Absolute, unless overridden such as with GIT_CONFIG_SYSTEM"
+    );
+    assert!(path.exists(), "Exists, since `git config` just found an entry there");
+}
+
 #[test]
 fn first_file_from_config_with_origin() {
     let macos =