gdb/testsuite: Remove duplicates from gdb.base/exprs.exp
When running the testsuite, I have:
Running .../gdb/testsuite/gdb.base/exprs.exp ...
DUPLICATE: gdb.base/exprs.exp: \$[0-9]* = red (setup)
Fix by using with_test_prefix where appropriate.
Tested on x86_64-linux.