gdb/testsuite: Remove duplicates from gdb.base/call-signal-resume.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/call-signal-resume.exp ...
    DUPLICATE: gdb.base/call-signal-resume.exp: dummy stack frame number
    DUPLICATE: gdb.base/call-signal-resume.exp: set confirm off
    DUPLICATE: gdb.base/call-signal-resume.exp: return

This is due to the fact that a pattern was probably copy/pasted to
re-use the logic while not adjusting the test names to avoid the
duplication.

Fix by removing the redundant tests ('set confirm off' only needs to be
used once) and adjusting the test names where appropriate.

Tested on x86_64-linux.