siso: fold the member-order flush test into the stale-non-dir table

TestFlush_MemberListedBeforeDirTarget repeated about 60 lines of
TestFlush_PopulatedDirReplacesStaleNonDir. The only real difference
was the order of the paths in the flush list. Make that order a table
dimension (dirTarget/membersFirst, crossed with file/symlink), so the
scenario becomes one table row instead of a second test body that
would drift out of sync over time.

Also use the got/want pattern for the value comparison in
TestOpenReadDeepPath.

Change-Id: I23571c1c486c156f1611db38b7497c483d43205b
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8037823
Reviewed-by: Yuke Liao <liaoyuke@google.com>
Auto-Submit: Philipp Wollermann <philwo@google.com>
Commit-Queue: Neri Marschik <nerima@google.com>