Test case for Bug 28308

The purpose of this test is described in the comments in
dprintf-execution-x-script.exp.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28308

The name of this new test was based on that of an existing test,
bp-cmds-execution-x-script.exp.  I started off by copying that test,
adding to it, and then rewriting almost all of it.  It's different
enough that I decided that listing the copyright year as 2021
was sufficient.