Run the tests where a feature is switched off

These six jobs only compiled. Nothing was stopping them from running, and what
they cover is not covered anywhere else: STDC_EXCEPTIONS is decided per
translation unit from __cpp_exceptions, and both options are private to the
library target, so the tests build the ordinary way and exercise a library
whose #ifdefs came out the other way. Popen's exception-free path had never
been executed by anything.

Checked before turning them on, in the configuration the jobs actually use,
which is the static default rather than the shared one the other jobs pass:
both options build and run all 190 cases on MSVC.

The cost is Boost.Test on three more platforms, which the shared setup action
already knows how to install.

Author: SineStriker

.github/workflows/ci.yml

@@ -87,6 +87,12 @@ jobs:
   #
   # Three platforms rather than five: the flag is chosen by MSVC against not-MSVC, and these
   # three are the MSVC standard library, libstdc++ and libc++.
+  #
+  # Both flags are private to the library target, so the tests are built the ordinary way and run
+  # against a library that was not. That is the same split a consumer gets, and it is the only
+  # thing here that reaches the library's exception-free branches at run time: STDC_EXCEPTIONS is
+  # worked out per translation unit from __cpp_exceptions, so the code the #ifdefs in popen and
+  # elsewhere select is compiled by these two jobs and by nothing else.
   no-features:
     name: ${{ matrix.platform }} ${{ matrix.config }}
     runs-on: ${{ matrix.os }}
@@ -118,11 +124,15 @@ jobs:
             no-exceptions) flags="-DSTDC_ENABLE_EXCEPTIONS=OFF" ;;
             no-rtti)       flags="-DSTDC_ENABLE_RTTI=OFF" ;;
           esac
-          cmake -G Ninja -S . -B build -DCMAKE_BUILD_TYPE=Debug -DSTDC_BUILD_TESTS=OFF $flags
+          cmake -G Ninja -S . -B build -DCMAKE_BUILD_TYPE=Debug -DSTDC_BUILD_TESTS=ON $flags
 
       - name: Build
         run: cmake --build build
 
+      - name: Test
+        shell: bash
+        run: ./build/bin/test_auto
+
   # MinGW builds the library but does not run the tests: Boost.Test for MinGW is its own
   # undertaking, and what is worth knowing here is that the Windows sources compile and link
   # against something other than the Microsoft toolchain.