Metric testcases fail with LANG=C

[dankamongmen]

[schwarzgerat](1) $ ./notcurses-tester -tc=Metric -p ../data/
Running with LANG=C
Running with TERM=xterm-kitty
Detected cell geometry: 132x64
[doctest] doctest version is "2.4.11"
[doctest] run with "--help" for options
===============================================================================
/home/dank/src/dankamongmen/notcurses/src/tests/metric.cpp:24:
TEST CASE:  Metric
  NegativePowersOfTen

/home/dank/src/dankamongmen/notcurses/src/tests/metric.cpp:442: FATAL ERROR: REQUIRE( ncqprefix(val, 1000000000000000ull, buf, '\0') ) is NOT correct!
  values: REQUIRE( nullptr )

===============================================================================
[doctest] test cases:   1 |   0 passed | 1 failed | 45 skipped
[doctest] assertions: 581 | 580 passed | 1 failed |
[doctest] Status: FAILURE!
[schwarzgerat](1) $ 

this is causing us trouble in pbuilder and should obviously be fixed either way.

[dankamongmen]

confirmed it works fine with LANG=en_US.UTF-8.

[dankamongmen]

as predicted, this has to do with µ being pulled from subprefixes, meaning we're using UTF8_SUBPREFIX rather than ASCII_SUBPREFIX.

[dankamongmen]

yep we're calling into ncmetric_use_utf8_internal()

[dankamongmen]

oh christ

[dankamongmen]

fixed, incoming, stupid

[dankamongmen]

so we were testing utf rather than *utf8, which was always true, and thus always enabling utf8 for ncmetric.