csis: Fix failing test_set_desired_size

Fixing typo issue in the test where instead of group size, the
sizeof(g_1) was tested.

Also since test_set_desired_size and test_get_desired_size does same,
this test was merged to test_set_get_desired_size

Bug: 262345129
Test: atest BluetoothInstrumentationTests
Test: atest bluetooth_csis_test
Tag: #feature
Change-Id: I34a42966f3457f98331fccfc933f64441de26ca5