gdb/testsuite: rename a proc and fix a typo

gdb/testsuite: rename a proc and fix a typo

Rename a proc in gdb.mi/user-selected-context-sync.exp, I think the
old name was most likely a typo.  The old name
match_re_or_ensure_not_output seems (to me) to imply we're in some way
checking that the regexp was not output.  But that's not what we are
doing, we're checking either for the regexp, or for no output, hence
the new name match_re_or_ensure_no_output.

Additionally, I found a definite typo in one of the comments that I've
also fixed.

I also updated some test names.  These tests (probably due to copy &
paste errors) has 'on MI' on their name, when they were actually
checking CLI output.  For these test I changed the name to use 'on
CLI'.

There should be no change in what is tested after this commit.