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>

ci/if_feat_check.vim

@@ -0,0 +1,15 @@
+if 1 " This prevents it from being run in tiny versions
+  " Check for required features
+  if exists("g:required")
+    for feature in g:required
+      if !has(feature)
+        echo "Error: Feature '" .. feature .. "' not found"
+        echo ''
+        cquit
+      endif
+    endfor
+    echo "\nChecked features: " .. string(g:required)
+    echo ''
+  endif
+endif
+" vim: sts=2 sw=2 et

ci/if_ver-1.vim

@@ -1,6 +1,6 @@
-" Print all interface versions for Ubuntu. Part 1.
+" Print all interface versions. Part 1.
 
-if 1
+if 1 " This prevents it from being run in tiny versions
   execute 'source' expand('<sfile>:h') .. '/if_ver-cmd.vim'
 
   echo "*** Interface versions ***\n"

ci/if_ver-2.vim

@@ -1,6 +1,8 @@
-" Print py3 interface versions for Ubuntu. Part 2.
+" Print py3 interface versions. Part 2.
+" This is done separately from part 1 because Vim cannot concurrently load
+" Python 2 and 3 together.
 
-if 1
+if 1 " This prevents it from being run in tiny versions
   execute 'source' expand('<sfile>:h') .. '/if_ver-cmd.vim'
 
   echo 'Python 3:'