iio: test: Fix GTS test config

The test config contained a copy-paste error. The IIO GTS helper test
was errorneously titled as "Test IIO formatting functions" in the
menuconfig.

Change the title of the tests to reflect what is tested.

Fixes: cf996f03 ("iio: test: test gain-time-scale helpers")
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://patch.msgid.link/Z0gt5R86WdeK73u2@mva-rohm


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>