testsuite: Fix false FAILs on too long base directory

I was getting

gu (print arg0)^M
= 0x7fffffffdafb
"/unsafebuild-x86_64-redhat-linux-gnu/gdb/testsuite.unix.-m64/outputs/gdb.guile/scm-value/scm-"...^M
(gdb) FAIL: gdb.guile/scm-value.exp: verify dereferenced value
python print (arg0)^M
0x7fffffffdafd
"/unsafebuild-x86_64-redhat-linux-gnu/gdb/testsuite.unix.-m64/outputs/gdb.python/py-value/py-v"...^M
(gdb) FAIL: gdb.python/py-value.exp: verify dereferenced value

and also:

(gdb) p argv[0]^M
$2 = 0x7fffffffd832 "/home/jkratoch/redhat/gdb-test-", 'x' <repeats 169
times>...^M
(gdb) FAIL: gdb.guile/scm-value.exp: argv[0] should be available on this
target

gdb/testsuite/ChangeLog
2016-01-11  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.guile/scm-value.exp (test_value_in_inferior): Set print elements
	and repeats to unlimited.
	* gdb.python/py-value.exp: Likewise.
	* lib/gdb.exp (gdb_has_argv0): Save and temporarily set print elements
	and repeats to unlimited.