document workaround test_bp2build_generates_all_buildfiles

test_bp2build_generates_all_buildfiles uses the same directory for
--output_base and --sandbox_tmpfs_path. This is required on the build
servers, but causes issues locally. This commit adds a more helpful
error message when this test fails.

Test: build/soong/tests/bp2build_bazel_test.sh
Change-Id: Ib29a7edf456f8ae39bd429ebb367c8115e02c284

tests/bp2build_bazel_test.sh

@@ -114,7 +114,10 @@ EOF
   fi
 }
 
+_save_trap=$(trap -p EXIT)
+trap '[[ $? -ne 0 ]] && echo Are you running this locally? Try changing --sandbox_tmpfs_path to something other than /tmp/ in build/bazel/linux.bazelrc.' EXIT
 test_bp2build_generates_all_buildfiles
+eval ${_save_trap}
 
 function test_cc_correctness {
   setup