Make gdb.python/py-inferior.exp test names unique

Make gdb.python/py-inferior.exp test names unique

Before we had:

      $ cat testsuite/gdb.sum | grep "PASS" | sort | uniq -c | sort -n
      ...
      1 PASS: gdb.python/py-inferior.exp: write str
      2 PASS: gdb.python/py-inferior.exp: Get inferior list length
      2 PASS: gdb.python/py-inferior.exp: py start_addr = gdb.selected_frame ().read_var ('search_buf')
      2 PASS: gdb.python/py-inferior.exp: Switch to first inferior
      3 PASS: gdb.python/py-inferior.exp: find mixed-sized pattern
      4 PASS: gdb.python/py-inferior.exp: py length = search_buf.type.sizeof
      4 PASS: gdb.python/py-inferior.exp: py start_addr = search_buf.address
      5 PASS: gdb.python/py-inferior.exp: Check inferior validity
      $

gdb/testsuite/ChangeLog:
2015-11-24  Pedro Alves  <palves@redhat.com>

	* gdb.python/py-inferior.exp: Use with_test_prefix.  Consistently
	use lowercase.