[gdb/testsuite] Fix duplicate test-names in gdb.dwarf2

[gdb/testsuite] Fix duplicate test-names in gdb.dwarf2

We currently have these duplicate test-names in gdb.dwarf2:
...
$ grep ^DUPLICATE: gdb.sum
DUPLICATE: gdb.dwarf2/comp-unit-lang.exp: show language
DUPLICATE: gdb.dwarf2/data-loc.exp: get integer valueof "sizeof (int)"
DUPLICATE: gdb.dwarf2/data-loc.exp: get integer valueof "sizeof (void *)"
DUPLICATE: gdb.dwarf2/data-loc.exp: get integer valueof "sizeof (int)"
DUPLICATE: gdb.dwarf2/data-loc.exp: get integer valueof "sizeof (int)"
DUPLICATE: gdb.dwarf2/data-loc.exp: ptype foo.array_type
DUPLICATE: gdb.dwarf2/varval.exp: get integer valueof "sizeof (int)"
DUPLICATE: gdb.dwarf2/varval.exp: get integer valueof "sizeof (void *)"
DUPLICATE: gdb.dwarf2/implref-struct.exp: print-object=off: \
  set print object off
DUPLICATE: gdb.dwarf2/implref-struct.exp: print-object=on: \
  set print object on
DUPLICATE: gdb.dwarf2/dw2-bad-parameter-type.exp: ptype f
...

Fix as appropriate.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-12  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/comp-unit-lang.exp: Use with_test_prefix.
	* gdb.dwarf2/dw2-bad-parameter-type.exp: Same.
	* gdb.dwarf2/implref-struct.exp: Same.
	* gdb.dwarf2/varval.exp: Ensure get_sizeof is called once per type.
	* gdb.dwarf2/data-loc.exp: Same.  Remove duplicate test.