gdb/testsuite: Remove duplicates from gdb.base/nested-subp2.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/nested-subp2.exp ...
    DUPLICATE: gdb.base/nested-subp2.exp: continue to the STOP marker
    DUPLICATE: gdb.base/nested-subp2.exp: print c
    DUPLICATE: gdb.base/nested-subp2.exp: print count

Fix by using with_test_prefix to differentiate the test that are
performed at different points during the execution of the debuggee.

Tested on x86_64-linux.
1 file changed