tests: Add support for adding internal tests

If the internal_tests option is enabled we build some internal tests
into the binary. These are added to the tests we run in testlibrary.

This is not intended to be enabled in production, as it adds size to
the real binary, but is useful for CI and development.

.github/workflows/check.yml

@@ -64,6 +64,7 @@ jobs:
           -Dgtkdoc=disabled \
           -Dhttp_backend=curl \
           -Dinternal_checks=true \
+          -Dinternal_tests=true \
           -Dsystem_dbus_proxy=xdg-dbus-proxy \
           _build
       env:
@@ -123,6 +124,7 @@ jobs:
           -Dgtkdoc=disabled \
           -Dhttp_backend=soup \
           -Dinternal_checks=true \
+          -Dinternal_tests=true \
           _build
       env:
         CFLAGS: -O2 -Wp,-D_FORTIFY_SOURCE=2