gdb/testsuite: Remove duplicates from gdb.base/checkpoint.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/checkpoint.exp ...
    DUPLICATE: gdb.base/checkpoint.exp: verify lines 5 two
    DUPLICATE: gdb.base/checkpoint.exp: restart 0 one

This patch fixes the various erroneous incorrect test names.

While at it, this patch also remove some trailing white spaces across
the file.

Tested on x86_64-linux.