tests: Check if program exists before using it

The tests/make-test-runtime.sh scripts sets '-e' in lieu of
implementing useful error checking, but doesn't actually check
if the programs it uses exist in the first place; and aborts
silently when they're not available.

It is more useful to warn about them, and stop execution earlier.
This leaves some leftover files, but arguably that's better than
leaving leftover files AND people scratching their heads.

Warn about missing required programs.

Related: https://github.com/flatpak/flatpak/issues/5020

tests/make-test-runtime.sh

@@ -47,6 +47,12 @@ add_bin() {
     local f=$1
     shift
 
+    # Check if the program is installed
+    if ! command -v "${f}" &> /dev/null; then
+        echo "${f} not found"
+        exit 1
+    fi
+
     if grep -qFe "${f}" $BINS; then
         # Already handled
         return 0