mktemp: fix error msg when suffix has path sep.

Correct the error message when the template argument contains a path
separator in its suffix. Before this commit:

    $ mktemp aXXX/b
    mktemp: too few X's in template 'b'

After this commit:

    $ mktemp aXXX/b
    mktemp: invalid suffix '/b', contains directory separator

This error message is more appropriate and matches the behavior of GNU
mktemp.

src/uu/mktemp/src/mktemp.rs

@@ -17,7 +17,7 @@ use std::env;
 use std::error::Error;
 use std::fmt::Display;
 use std::iter;
-use std::path::{is_separator, Path, PathBuf};
+use std::path::{is_separator, Path, PathBuf, MAIN_SEPARATOR};
 
 use rand::Rng;
 use tempfile::Builder;
@@ -129,6 +129,19 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
             };
             let filename = path.file_name();
             let template = filename.unwrap().to_str().unwrap();
+            // If the command line was `mktemp aXXX/b`, then we will
+            // find that `tmp`, which is the result of getting the
+            // parent when treating the argument as a path, contains
+            // at least three consecutive Xs. This means that there
+            // was a path separator in the suffix, which is not
+            // allowed.
+            if tmp.display().to_string().contains("XXX") {
+                return Err(MkTempError::SuffixContainsDirSeparator(format!(
+                    "{}{}",
+                    MAIN_SEPARATOR, template
+                ))
+                .into());
+            }
             (template, tmp)
         }
     } else {

tests/by-util/test_mktemp.rs

@@ -496,3 +496,18 @@ fn test_template_path_separator() {
             "a/bXXX".quote()
         ));
 }
+
+/// Test that a suffix with a path separator is invalid.
+#[test]
+fn test_suffix_path_separator() {
+    #[cfg(not(windows))]
+    new_ucmd!()
+        .arg("aXXX/b")
+        .fails()
+        .stderr_only("mktemp: invalid suffix '/b', contains directory separator\n");
+    #[cfg(windows)]
+    new_ucmd!()
+        .arg(r"aXXX\b")
+        .fails()
+        .stderr_only("mktemp: invalid suffix '\\b', contains directory separator\n");
+}