Clarify assert and expect messages

This rewords some messages in recently added tests to use "It ..."
in a way that people reading test run output are likely to find
intuitive, using it in places where it will be clear, and rewording
to remove it in one place where it would be misleading.

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

@@ -362,13 +362,16 @@ use serial_test::serial;
 fn check_exe_info() {
     let path = super::exe_info()
         .map(crate::from_bstring)
-        .expect("Nonempty config in the test environment");
+        .expect("It is present in the test environment (nonempty config)");
 
     assert!(
         path.is_absolute(),
-        "Absolute, unless overridden such as with GIT_CONFIG_SYSTEM"
+        "It is absolute (unless overridden such as with GIT_CONFIG_SYSTEM)"
+    );
+    assert!(
+        path.exists(),
+        "It should exist on disk, since `git config` just found an entry there"
     );
-    assert!(path.exists(), "Exists, since `git config` just found an entry there");
 }
 
 #[test]
@@ -380,7 +383,7 @@ fn exe_info() {
 #[test]
 #[serial]
 fn exe_info_tolerates_broken_tmp() {
-    let empty = gix_testtools::tempfile::tempdir().expect("We can create a new temporary subdirectory");
+    let empty = gix_testtools::tempfile::tempdir().expect("can create new temporary subdirectory");
     let nonexistent = empty.path().join("nonexistent");
     assert!(!nonexistent.exists(), "Test bug: Need nonexistent directory");
     let nonexistent_str = nonexistent.to_str().expect("valid Unicode");
@@ -426,7 +429,7 @@ fn exe_info_never_from_local_scope_even_if_temp_is_here() {
     assert_eq!(
         std::env::temp_dir(),
         repo,
-        "It is likely that setting up the test failed"
+        "Possible test bug: Setting up the test may have failed"
     );
     let maybe_path = super::exe_info();
     assert_eq!(