[test] Move cctest/test-conversions to unittests

... /numbers/conversions-unittest.

Bug: v8:12781
Change-Id: I81043f8bcebf5ce1292111211af1bea297c9eea4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3604962Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: 王澳 <wangao.james@bytedance.com>
Cr-Commit-Position: refs/heads/main@{#80172}

test/cctest/BUILD.gn

@@ -197,7 +197,6 @@ v8_source_set("cctest_sources") {
     "test-code-stub-assembler.cc",
     "test-concurrent-transition-array.cc",
     "test-constantpool.cc",
-    "test-conversions.cc",
     "test-cpu-profiler.cc",
     "test-debug-helper.cc",
     "test-debug.cc",