exit integration test successfully if the crate build failed before applying rustfmt

The `cargo test --all` command failed and exited the main process with a
SIGINT. Trapping the signal or trying to get the code of a subshell
didn't work.

Close rust-lang#2724

Author: scampi

ci/integration.sh

@@ -42,8 +42,8 @@ function check_fmt_with_lib_tests {
 
 function check_fmt_base {
     local test_args="$1"
-    cargo test $test_args
-    if [[ $? != 0 ]]; then
+    local build=$(cargo test $test_args 2>&1)
+    if [[ "$build" =~ "build failed" ]]; then
           return 0
     fi
     touch rustfmt.toml