Copy browser test output to file

BUG=
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Review URL: https://codereview.chromium.org/2132593003 .

Author: leafpetersen

pkg/dev_compiler/tool/browser_test.sh

@@ -34,4 +34,4 @@ if [[ "$OSTYPE" == "linux-gnu" ]] && [[ -z "$CHROME_CANARY_BIN" ]]; then
 fi
 
 npm install
-npm test
+npm test | tee test_output.txt