[core] Switch test build to googletest git submodule

scripts/vendor/gtest.sh

@@ -1,19 +0,0 @@
-#!/usr/bin/env bash
-source "$(dirname "${BASH_SOURCE[0]}")/common.sh"
-
-NAME=gtest
-VERSION=1.8.0
-ROOT=googletest-release-1.8.0/googletest
-
-download "https://github.com/google/googletest/archive/release-$VERSION.tar.gz"
-init
-STRIP_COMPONENTS=2 extract_gzip --exclude "gtest-all.cc" --exclude "gtest_main.cc" "$ROOT/include/*.h" "$ROOT/src" "$ROOT/LICENSE"
-
-for FILE in $(find include src -name "*.h" -o -name "*.cc"); do
-    sed -e 's/^#include \"src\/\([^\"]\{1,\}\)\"/#include "\1"/' \
-        -e 's/^#include \"\(gtest\/[^\"]\{1,\}\)\"/#include <\1>/' \
-        "$FILE" > "$FILE.bak" && mv "$FILE.bak" "$FILE"
-done
-
-file_list include src -name "*.h" -o -name "*.cc"
-