[gdb/testsuite] Fix test names in gdb.base/longjmp.exp

[gdb/testsuite] Fix test names in gdb.base/longjmp.exp

When running test-case gdb.base/longjmp.exp, we have:
...
PASS: gdb.base/longjmp.exp: next over setjmp (1)
  ...
PASS: gdb.base/longjmp.exp: next over setjmp (2)
...

The trailing " (1)" and " (2)" are interpreted as comments rather than parts
of the test name, and therefore this is a duplicate, which is currently not
detected by our duplicate detection mechanism (PR testsuite/29772).

Fix the duplicate by using with_test_prefix.

Tested on x86_64-linux.