Escape the $s in include/show_slave_status.inc’s error messages

[ParadoxV5]

• The Jira issue number for this PR is: MDEV-______

Description

--die performs variable substitution like --echo does, so the variable names were parsed rather than mentioned verbatim.

Before Patch (Erroneous)

At line 89: Bug in test case: The mysqltest variable  is not set.
At line 98: Bug in test case: Both 1 and foobar are set.

After Patch (Intended)

At line 89: Bug in test case: The mysqltest variable $status_items is not set.
At line 98: Bug in test case: Both $all_slaves_status and $slave_name are set.

Release Notes

N/A
Unless there are users who write their own Replication MTR tests.

How can this PR be tested?

We are about as reliant on include/*.inc files as actual MTR commands.
This means the include/*.incs are as eligible for unit testing as MTR commands are, right?

PR quality check

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.
• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.