gdb/testsuite: remove use of then keyword from gdb.*/*.exp scripts

gdb/testsuite: remove use of then keyword from gdb.*/*.exp scripts

The canonical form of 'if' in modern TCL is 'if {} {}'.  But there's
still a bunch of places in the testsuite where we make use of the
'then' keyword, and sometimes these get copies into new tests, which
just spreads poor practice.

This commit removes all use of the 'then' keyword from the remaining
gdb.*/*.exp scripts.  Previous commits have done the bulk of this
removal, this commit just handles the remaining directories that each
contain a low number of instances.

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