mktemp: only replace last contiguous block of Xs

Fix a bug in which `mktemp` would replace everything in the template
argument from the first 'X' to the last 'X' with random bytes, instead
of just replacing the last contiguous block of 'X's.

Before this commit,

    $ mktemp XXX_XXX
    2meCpfM

After this commit,

    $ mktemp XXX_XXX
    XXX_Rp5

This fixes test cases `suffix2f` and `suffix2d` in
`tests/misc/mktemp.pl` in the GNU coreutils test suite.

tests/by-util/test_mktemp.rs

@@ -572,3 +572,25 @@ fn test_too_few_xs_suffix_directory() {
 fn test_too_many_arguments() {
     new_ucmd!().args(&["-q", "a", "b"]).fails().code_is(1);
 }
+
+#[test]
+fn test_two_contiguous_wildcard_blocks() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let template = "XXX_XXX";
+    let result = ucmd.arg(template).succeeds();
+    let filename = result.no_stderr().stdout_str().trim_end();
+    assert_eq!(&filename[..4], "XXX_");
+    assert_matches_template!(template, filename);
+    assert!(at.file_exists(filename));
+}
+
+#[test]
+fn test_three_contiguous_wildcard_blocks() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let template = "XXX_XXX_XXX";
+    let result = ucmd.arg(template).succeeds();
+    let filename = result.no_stderr().stdout_str().trim_end();
+    assert_eq!(&filename[..8], "XXX_XXX_");
+    assert_matches_template!(template, filename);
+    assert!(at.file_exists(filename));
+}