Inserting an Inside Horizontal Border on a single-line paragraph

The solution using a tab leader is the way to go; there's just one more step, as follows:

1. In the paragraph in question, set a center-aligned tab stop centered between the margins.
2. Set a right-aligned tab stop at the right margin.
3. In the Tabs dialog, select a hyphen tab leader for both tab stops.
4. Press Tab once, type "TESTS"; then press Tab again. You will have a broken line before and after the word.
5. Select the line or tab character and press Ctrl+D to open the Font dialog.
6. On the Advanced tab, in the Spacing box, select Condensed. The default setting is 1 point, but you can increase this as needed to make the hyphens join. You may also want to increase the point size of the tab characters to make the line heavier.
7. You may want to add a space character before and after the text so that the line is not so close.

Here's an example:

Another approach is to format the tab characters as Strikethrough; again, you may need to increase the point size.

Another approach that comes to my mind sounds complicated but is rather easy. It's based on a page-wide table with two rows and three columns. Hide the outer borders but keep the horizontal divider between the two rows visible. Now combine both cells of the middle column. This is where you write your Text ("Tests"). The result looks like your screenshot.