[gimp/wip/Jehan/ninja-test] build: use POSIX test syntax.

[Jehan <jehanp src gnome org>]

commit a4b62f1e037ff38869bcfab469d30294a2cf9d1b
Author: Jehan <jehan girinstud io>
Date:   Sun Nov 14 17:35:57 2021 +0100

    build: use POSIX test syntax.
    
    The [[ ]] is not POSIX, hence less portable.
    I see our `ninja test` calls in Gitlab CI output a:
    
    > /builds/GNOME/gimp/build/meson/run_test_env.sh: 7: [[: not found
    
    (though it doesn't break the tests)

 build/meson/run_test_env.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/build/meson/run_test_env.sh b/build/meson/run_test_env.sh
index f71585a432..19cb191052 100644
--- a/build/meson/run_test_env.sh
+++ b/build/meson/run_test_env.sh
@@ -4,7 +4,7 @@
 #
 # Define the "UI_TEST" for all tests that should run headless
 
-if [[ -n "${UI_TEST}" ]]; then
+if [ -n "${UI_TEST}" ]; then
   # Use Xvfb to simulate a graphical session; note that this needs
   # a new enough version which has the -d option.
   #