ci: Check and confirm Vim feature flags exist before testing

Vim tests for features such as python3 relies on checking the feature
flag exists by doing `has('python3')`. However, if the feature itself is
broken and the flag returns 0, the relevant tests will simply silently
get ignored and CI will passed erroneously. As a preventive measure, as
basic checks to make sure certain feature flags are correct as a basic
smoke test.

Currently only checking two types of feature flags:

1. Features that depend on system packages being installed properly
   (e.g. sodium) and could be erroneously dropped if the CI environment
   changed or a bug exists in the configure script.
2. Scripting languages. When in dynamic mode, these feature flags (e.g.
   "ruby", "python3") will return 0 when the lib cannot be found or the
   code has an initialization bug. This happened in #16964 where CI
   still passed despite Python 3 being broken.

closes: #16998

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Filelist

@@ -23,6 +23,7 @@ SRC_ALL =	\
 		ci/appveyor.bat \
 		ci/config.mk*.sed \
 		ci/if_ver*.vim \
+		ci/if_feat_check.vim \
 		ci/setup-xvfb.sh \
 		ci/remove_snap.sh \
 		src/Make_all.mak \