fastboot: Introduce ParseNetworkSerial unit tests

Introduced positive and negative unit tests to cover
ParseNetworkSerial logic. Alongside with that move
result related stuff to the separate header.

Test: atest fastboot_test
Test: manually checked basic functionality works fine
Bug: 271155012
Change-Id: Icac6053c11b5a36daa64555209555826ea28cc61
Signed-off-by: Dmitrii Merkurev <dimorinny@google.com>