Add `skip_if_not_installed("vdiffr")` to the 3 tests below

[Yuri05]

== Skipped tests ===============================================================================================================
* vdiffr cannot be loaded (8)

== Failed tests ================================================================================================================
-- Error (test-font-label.R:57): Long texts are properly handled in labels -----
<packageNotFoundError/error/condition>
Error in `loadNamespace(x)`: there is no package called ‘vdiffr’
Backtrace:
    x
 1. \-base::loadNamespace(x) at test-font-label.R:57:2
 2.   \-base::withRestarts(stop(cond), retry_loadNamespace = function() NULL)
 3.     \-base (local) withOneRestart(expr, restarts[[1L]])
 4.       \-base (local) doWithOneRestart(return(expr), restart)
-- Error (test-font-label.R:95): Markdown syntax is not supported because Label and Font overwrite its with the FontFace argument --
<packageNotFoundError/error/condition>
Error in `loadNamespace(x)`: there is no package called ‘vdiffr’
Backtrace:
    x
 1. \-base::loadNamespace(x) at test-font-label.R:95:2
 2.   \-base::withRestarts(stop(cond), retry_loadNamespace = function() NULL)
 3.     \-base (local) withOneRestart(expr, restarts[[1L]])
 4.       \-base (local) doWithOneRestart(return(expr), restart)
-- Error (test-plot-grid.R:104): Plot Grid can have very long texts and contain plots with very long titles --
<packageNotFoundError/error/condition>
Error in `loadNamespace(x)`: there is no package called ‘vdiffr’
Backtrace:
    x
 1. \-base::loadNamespace(x) at test-plot-grid.R:104:2
 2.   \-base::withRestarts(stop(cond), retry_loadNamespace = function() NULL)
 3.     \-base (local) withOneRestart(expr, restarts[[1L]])
 4.       \-base (local) doWithOneRestart(return(expr), restart)

[ FAIL 3 | WARN 0 | SKIP 8 | PASS 329 ]