tests: avoid FP failure due to exhausted memory

* tests/long-line-vs-2GiB-read: Don't declare the test "failed"
when running out of memory.  In that case, skip it.
pcre: tell grep -P to relax its stance on invalid multibyte chars

Do not exit-2 for invalid UTF-8 characters.  Just prior to this
change, this command would match no lines and fail like this:
  $ printf 'j\x82\nj\n'|LC_ALL=en_US.UTF-8 grep -P j|cat -A; echo $?
  grep: invalid UTF-8 byte sequence in input
  2
After this change, the same command matches both lines, and succeeds:
  jM-^B$
  j$
  0
* src/pcresearch.c (Pcompile): Use PCRE_NO_UTF8_CHECK, too, and
add a comment.
* tests/pcre-utf8: Add a test and a comment.
This change did not work with Debian unstable pcre-8.31-2
or with some 8.33 and 8.34-based versions, but does work with
Fedora 20's 8.33 and with a built-from-latest source library.
Based on a patch by Santiago Ruano Rincón.
See http://bugs.gnu.org/15758/