gdb/testsuite: resolve some duplicate test names in gdb.base

gdb/testsuite: resolve some duplicate test names in gdb.base

This commit resolves all the duplicate test names that I see in the
script:

  gdb.base/attach-pie-misread.exp

The duplicate names all come from a second call to
build_executable_own_libs, so in this commit I've places the second
call inside a with_test_prefix block.

While I was making this change I've also modified the value being
passed as the testname for the second build_executable_own_libs call.
Previously we used ${test}, however, I think this was likely a
mistake, the 'test' variable is setup for the previous test.  I
suspect that ${testfile} is a better choice - especially now we have a
testname prefix.

As the testname is only used (after various calls) from within
build_executable_from_specs should the build fail, I don't think this
change really makes much difference though.

There should be no change in what is tested after this commit.