Tests: Fix escaping for error message in create block test

[gziolo]

What and Why?

Addresses the issue raised by @mcsf in #39049 (comment):

In sh and bash, backticks are the old way to represent command substitution. Which means that the line above actually evals as:

error "Expected $expected files in the $(src) directory ..."

which will invoke src as a command that it will try to find in $PATH, which will either fail or do something that we don't want.

How?

The fix should be to escape:

error "Expected $expected files in the \`src\` directory, but found $actual."

The same applies for this other occurrence in the file:

error "Expected $expected files in the `build` directory, but found $actual."

Testing Instructions

CI should still pass. You can test it locally by changing bin/test-create-block.sh in a way that prints the modified error.

[mcsf]

Thanks!