Support expect from different source (RDT-311)

[hfudev]

Now we have a few different sources, like serial, openocd, gdb, ..., etc. Each source is a single process, and would have their own output and input. if we combine all the output from different sources into one single pexpect process, then it would sometimes be messed up.

Success Criteria:

• Record the output of each process into a single file, and has its own pexpect process.
• modify the original PexpectProcess class, to make it as the parent class to hold all the processes created by different sources. The users would still use the same API to expect the patterns from different sources.
• add keyword source of expect functions to indicate which source to expect from. default set to serial.
• add keyword source of redirect fixture
• add attr source to all derived fixture classes

and one reminder:

• don't make breaking changes. it's not necessary.