libdm: improve ParseStatusText() and test it

Simplify the argument parsing of DmTargetSnapshot::ParseStatusText() and
improve its robustness when dealing with wrong imputs.

Add test for DmTargetSnapshot::ParseStatusText().

Change-Id: I7f078c9ecacb402e71db49e3e7072e37cffbc234
Test: dm_test
Signed-off-by: Alessio Balsini <balsini@google.com>