Fix FFI tests

[Manishearth]

Turns out we weren't running them in a way that caused their failure to fail CI. oops.

[codecov-commenter]

Codecov Report

Merging #889 (b624649) into main (09d5aef) will increase coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #889      +/-   ##
==========================================
+ Coverage   74.32%   74.40%   +0.08%     
==========================================
  Files         206      206              
  Lines       13042    13092      +50     
==========================================
+ Hits         9693     9741      +48     
- Misses       3349     3351       +2     

Impacted Files	Coverage Δ
ffi/capi/src/locale.rs	0.00% <0.00%> (ø)
provider/blob/src/export/blob_exporter.rs	94.44% <0.00%> (-2.78%)	⬇️
components/uniset/src/uniset.rs	98.93% <0.00%> (-1.07%)	⬇️
ffi/capi/src/provider.rs	0.00% <0.00%> (ø)
components/decimal/src/lib.rs	94.44% <0.00%> (ø)
components/plurals/src/lib.rs	86.95% <0.00%> (ø)
provider/blob/src/path_util.rs	100.00% <0.00%> (ø)
components/decimal/src/error.rs	0.00% <0.00%> (ø)
components/plurals/src/error.rs	33.33% <0.00%> (ø)
components/plurals/src/operands.rs	90.90% <0.00%> (ø)
... and 10 more

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[coveralls]

Pull Request Test Coverage Report for Build 7950f4fa5948b81aa2fb11b3e0230e191c3465f6-PR-889

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 74.479%

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Totals
Change from base Build 9acd1e99b12cc2c3eac2338fbe07f0059686b6e7:	0.008%
Covered Lines:	9873
Relevant Lines:	13256

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💛 - Coveralls

[Manishearth]

mac:

./../../include/locale.hpp:23:8: error: no template named 'optional' in namespace 'std'
  std::optional<std::string> ToString() const {
  ~~~~~^
./../../include/locale.hpp:30:12: error: no viable conversion from returned value of type 'std::string' (aka 'basic_string<char, char_traits<char>, allocator<char> >') to function return type 'int'
    return out;
           ^~~

windows:

gcc test.c ../../../../target/debug/libicu_capi.a -ldl -lpthread -lm -g
gcc.exe: error: ../../../../target/debug/libicu_capi.a: No such file or directory

Looks like our CAPI tests have been broken on other platforms for a while. I'm thinking of splitting our FFI tests onto a separate ubuntu-only builder for now

[Manishearth]

Once this lands I can file followups to get it to build on Mac and Windows