Skip compositor tests without recording a failure

Two problems made a missing Wayland compositor look like broken tests.

The cleanup traps ended on a bare conditional, so when a test skipped
before creating its TMPDIR the trap's last command returned 1 and, under
set -e, that overrode the explicit exit 0.

Six tests that launch quickshell had no compositor guard at all, so they
ran anyway and failed on the Qt platform plugin.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/shell.d/button-border-stability-test.sh

@@ -17,6 +17,11 @@ assert(
 )
 JS
 
+if [[ -z ${WAYLAND_DISPLAY:-} ]]; then
+  pass "no Wayland compositor; skipping Button hover geometry runtime test"
+  exit 0
+fi
+
 if ! command -v quickshell >/dev/null 2>&1; then
   pass "quickshell not installed; skipping Button hover geometry runtime test"
   exit 0
@@ -24,7 +29,9 @@ fi
 
 TMPDIR=$(mktemp -d)
 cleanup() {
-  [[ -d $TMPDIR ]] && rm -rf "$TMPDIR"
+  if [[ -d $TMPDIR ]]; then
+    rm -rf "$TMPDIR"
+  fi
 }
 trap cleanup EXIT