Fix gdb.ada/assign_arr.exp by using more unique names.

Fix gdb.ada/assign_arr.exp by using more unique names.

The test fails (timeout) due to conflict between var 'input' and s-ststop.adb 'input':
(gdb) print input.u2 := (0.25,0.5,0.75)
Multiple matches for input
[0] cancel
[1] system.strings.stream_ops.storage_array_ops.input (access ada.streams.root_stream_type; system.strings.stream_ops.io_kind; natural) return system.storage_elements.storage_array at s-ststop.adb:127
[2] system.strings.stream_ops.stream_element_array_ops.input (access ada.streams.root_stream_type; system.strings.stream_ops.io_kind; natural) return ada.streams.stream_element_array at s-ststop.adb:127
[3] system.strings.stream_ops.string_ops.input (access ada.streams.root_stream_type; system.strings.stream_ops.io_kind; natural) return string at s-ststop.adb:127
[4] system.strings.stream_ops.wide_string_ops.input (access ada.streams.root_stream_type; system.strings.stream_ops.io_kind; natural) return wide_string at s-ststop.adb:127
[5] system.strings.stream_ops.wide_wide_string_ops.input (access ada.streams.root_stream_type; system.strings.stream_ops.io_kind; natural) return wide_wide_string at s-ststop.adb:127
[6] target_wrapper.input at /bd/home/philippe/gdb/git/info_t/gdb/testsuite/gdb.ada/assign_arr/target_wrapper.ads:24
> FAIL: gdb.ada/assign_arr.exp: print input.u2 := (0.25,0.5,0.75) (timeout)

gdb/testsuite/ChangeLog
2018-12-26  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.ada/assign_arr/target_wrapper.ads (Input): Rename to
	Assign_Arr_Input.
	main_p324_051.adb: Update accordingly.
	gdb.ada/assign_arr.exp: Likewise.