tests: (column) use actually invalid multibytes to test encoding

tests: (column) use actually invalid multibytes to test encoding

If reading an invalid multibyte sequence, column(1) will encode the byte
as "\x<hex>" instead. The tests try to verify that by piping "£" into
column(1). As the tests run with LC_ALL=POSIX by default, though, libc
implementations strictly adhering to the POSIX standard will treat all
characters as valid multibyte characters. As a consequence, no EILSEQ is
raised by mbtowc(3P) and the character is not encoded as hex, breaking
the test.

Fix this by setting LC_ALL=C.UTF-8. As "£" is a valid UTF-8 character,
we also change the test to use a proper illegal multibyte sequence.

Signed-off-by: Patrick Steinhardt <ps@pks.im>