tests/drivers/clock_control: stm32_common_devices: lptim check disabled

The test checks if the peripheral gating clock was correctly disabled
after the test, but accidentally the I2C_CLK was checked instead of the
LPTIM_CLK.

This commit fixes this by using __HAL_RCC_LPTIM1_IS_CLK_ENABLED instead.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>