tests: drivers: uart: config api fix dependency #23459

Tests were added for the UART configure API in #22849. The existing test
implementation works, but suggested to reduce dependency between test
cases. This commit allows each test to run independently of each other
by removing the function call of uart_config_get() in the
uart_configure() test case, and uses the uart_config_get() test case to
confirm the value set and gotten/read.

Fixes #23459

Signed-off-by: Jennifer Williams <jennifer.m.williams@intel.com>