Add groundwork to easily all-enable bazel-mode

This will make it easy to change a "false" boolean to "true" to push
`--bazel-mode` by default for all users.

Users may disable bazel-by-default with BUILD_BROKEN_DISABLE_BAZEL, and
bazel is disabled by default on Darwin hosts (due to lack of test
coverage).

Bug: 254628592
Bug: 254084584
Test: Presubmits
Test: Presubmits for aosp/2263623
Change-Id: Iec712119e06ca8ec93028207d88277f879184cc2
diff --git a/tests/mixed_mode_test.sh b/tests/mixed_mode_test.sh
index 076ec4b..8949b42 100755
--- a/tests/mixed_mode_test.sh
+++ b/tests/mixed_mode_test.sh
@@ -12,7 +12,6 @@
 
 function test_bazel_smoke {
   setup
-  create_mock_bazel
 
   run_soong bp2build
 
@@ -21,7 +20,6 @@
 
 function test_add_irrelevant_file {
   setup
-  create_mock_bazel
 
   mkdir -p soong_tests/a/b
   touch soong_tests/a/b/c.txt
@@ -33,7 +31,7 @@
 }
 EOF
 
-  run_soong --bazel-mode nothing
+  run_soong --bazel-mode-staging nothing
 
   if [[ ! -e out/soong/bp2build/soong_tests/a/b/BUILD.bazel ]]; then
     fail "BUILD.bazel not created"
@@ -48,7 +46,7 @@
 
   touch soong_tests/a/irrelevant.txt
 
-  run_soong --bazel-mode nothing
+  run_soong --bazel-mode-staging nothing
   local mtime_build2=$(stat -c "%y" out/soong/bp2build/soong_tests/a/b/BUILD.bazel)
   local mtime_ninja2=$(stat -c "%y" out/soong/build.ninja)