ci: syntax tests spam output

ci: syntax tests spam output

Currently syntax tests outputs all the Vim commands to the console,
which is annoying for a local developer but also makes the CI output
impossible to parse and just printing all the terminal output comes with
a performance penalty.

So let's just simply redirect all output to /dev/null similar to what
the script tests do.

This can be turned off locally for someone diagnosing issues but it
should not be the default behavior to output all terminal control
sequences to output in CI.

closes: #16612

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