gdb/testsuite: explicitly test for stderr in gdb.mi/mi-dprintf.exp

gdb/testsuite: explicitly test for stderr in gdb.mi/mi-dprintf.exp

As mentioned in commit 3f5bbc3e2075ef5061a815c73fdc277218489f22, some
compilers such as clang don't add debug information about stderr by
default, leaving it to external debug packages.

This commit adds a way to check if GDB has access to stderr information
when in MI mode, and uses this new mechanism to skip the related section
of the test gdb.mi/mi-dprintf.exp. It also fixes an incorrect name for a
test in that file.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Kevin Buettner <kevinb@redhat.com>