releasetools: Reduce the memory use in test_common.py.

test_common constructs a few 2GiB strings in memory, which leads to huge
memory footprint (18GiB). This CL moves away from in-memory strings to
generators, which reduces the memory use down to 41MiB. It also reduces
the time cost from 294s to 139s as an extra benefit for free.

The CL addresses some trivial pylint warnings as well.

* Before

$ /usr/bin/time -v python -m unittest -v test_common
...
----------------------------------------------------------------------
Ran 11 tests in 294.986s

OK
	Command being timed: "python -m unittest -v test_common"
	User time (seconds): 110.51
	System time (seconds): 109.34
	Percent of CPU this job got: 74%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 4:55.06
	Average shared text size (kbytes): 0
	Average unshared data size (kbytes): 0
	Average stack size (kbytes): 0
	Average total size (kbytes): 0
	Maximum resident set size (kbytes): 18894172
	Average resident set size (kbytes): 0
	Major (requiring I/O) page faults: 1
	Minor (reclaiming a frame) page faults: 20774908
	Voluntary context switches: 48
	Involuntary context switches: 3241
	Swaps: 0
	File system inputs: 184
	File system outputs: 8406424
	Socket messages sent: 0
	Socket messages received: 0
	Signals delivered: 0
	Page size (bytes): 4096
	Exit status: 0

* After

$ /usr/bin/time -v python -m unittest -v test_common
...
----------------------------------------------------------------------
Ran 11 tests in 139.100s

OK
	Command being timed: "python -m unittest -v test_common"
	User time (seconds): 59.00
	System time (seconds): 4.73
	Percent of CPU this job got: 45%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 2:19.17
	Average shared text size (kbytes): 0
	Average unshared data size (kbytes): 0
	Average stack size (kbytes): 0
	Average total size (kbytes): 0
	Maximum resident set size (kbytes): 41252
	Average resident set size (kbytes): 0
	Major (requiring I/O) page faults: 0
	Minor (reclaiming a frame) page faults: 106569
	Voluntary context switches: 44
	Involuntary context switches: 103
	Swaps: 0
	File system inputs: 8
	File system outputs: 8422808
	Socket messages sent: 0
	Socket messages received: 0
	Signals delivered: 0
	Page size (bytes): 4096
	Exit status: 0

Fixes: 68988396
Test: See above.
Change-Id: I00f16603a4ee59fb085b189c6f5b5ee9d2378690
1 file changed