[wptrunner] Generate `{actual,expected}_text` with trailing newline

`run_{web,wpt}_tests.py` should produce the exact same testharness
output so that an `-expected.txt` generated by one runs expectedly with
the other (either locally or a test moved between CI suites).

Most testharness `-expected.txt` in the tree already include the
trailing newline, so standardize in that direction, not without a
trailing newline.

Fixed: 383572679
Change-Id: Id01776f614919eeabeb6eb7ec50a190ac1609192
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6152660
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1403329}