check-exercises: Show all failures in DENYWARNINGS mode

This requires not setting -e for the `cargo test` run, nor exiting early
in the manual status check.

The motivation is that we'd like to see all warnings in this mode, not
just the first exercise with warnings.

Author: petertseng

_test/check-exercises.sh

@@ -1,9 +1,16 @@
 #!/bin/bash
 
-set -e
+# In DENYWARNINGS mode, do not set -e so that we run all tests.
+# This allows us to see all warnings.
+if [ -z "$DENYWARNINGS" ]; then
+    set -e
+fi
+
 tmp=${TMPDIR:-/tmp/}
 mkdir "${tmp}exercises"
 
+exitcode=0
+
 # An exercise worth testing is defined here as any top level directory with
 # a 'tests' directory
 for exercise in exercises/*/tests; do
@@ -38,7 +45,8 @@ for exercise in exercises/*/tests; do
 
     if [ $status -ne 0 ]
     then
-        echo "Failed";
-        return 1;
+        exitcode=1
     fi
 done
+
+exit $exitcode