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.