Change symlink_forest to use relative symlinks.

Also add script to remove the Bazel output base.
This will assist with supporting movable checkouts alongside
mixed builds.

Bug: 259191764
Test: m && (move topic and prepare_moved_top.sh) && m
Test: m && prepare_moved_top.sh && m
Test: build/soong/tests/relative_symlinks_test.sh

Change-Id: I0f53da8d99f752fad496cf3ac61b01f001b7296d

tests/lib.sh

@@ -8,10 +8,15 @@ HARDWIRED_MOCK_TOP=
 
 REAL_TOP="$(readlink -f "$(dirname "$0")"/../../..)"
 
+function make_mock_top {
+  mock=$(mktemp -t -d st.XXXXX)
+  echo "$mock"
+}
+
 if [[ -n "$HARDWIRED_MOCK_TOP" ]]; then
   MOCK_TOP="$HARDWIRED_MOCK_TOP"
 else
-  MOCK_TOP=$(mktemp -t -d st.XXXXX)
+  MOCK_TOP=$(make_mock_top)
   trap cleanup_mock_top EXIT
 fi
 
@@ -197,3 +202,10 @@ function scan_and_run_tests {
     info "Completed test case \e[96;1m$f\e[0m"
   done
 }
+
+function move_mock_top {
+  MOCK_TOP2=$(make_mock_top)
+  mv $MOCK_TOP $MOCK_TOP2
+  MOCK_TOP=$MOCK_TOP2
+  trap cleanup_mock_top EXIT
+}