[test] Move cctest/test-bignum to unittests

... /base/bignum-unittest.

Bug: v8:12781
Change-Id: I217dd7c4ef59cf1fdbe83a268d0288c38f0e9d4d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3599401Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: 王澳 <wangao.james@bytedance.com>
Cr-Commit-Position: refs/heads/main@{#80138}

test/cctest/BUILD.gn

@@ -194,7 +194,6 @@ v8_source_set("cctest_sources") {
     "test-api.h",
     "test-atomicops.cc",
     "test-bignum-dtoa.cc",
-    "test-bignum.cc",
     "test-circular-queue.cc",
     "test-code-layout.cc",
     "test-code-stub-assembler.cc",

test/unittests/BUILD.gn

@@ -215,6 +215,7 @@ v8_source_set("unittests_sources") {
     "api/v8-object-unittest.cc",
     "base/address-region-unittest.cc",
     "base/atomic-utils-unittest.cc",
+    "base/bignum-unittest.cc",
     "base/bits-unittest.cc",
     "base/cpu-unittest.cc",
     "base/division-by-constant-unittest.cc",