[gdb/testsuite] Simplify gdb.ada/mi_ex_cond.exp

Simplify gdb.ada/mi_ex_cond.exp using the usual tools: subst_vars, quotemeta,
{}.

Also simplify variable syntax: "${::foo}" -> $foo.

The one case where I used subst_vars was where quotemeta forced me to write
out the @DECIMAL expression inside @/.../.  I've filed a PR about this [2].

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=33900