[wasm] Refactor mjsunit atomics tests into smaller tests

Tests were monolithic, with large loops, and slow on ARM64. Refactor to
small tests so they can be sharded better, reduce page size to 1 to
keep the loops small.

BUG=v8:6532

Change-Id: I712551564d4a70fc12acdf114922feb614aeb271
Reviewed-on: https://chromium-review.googlesource.com/611614Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47353}

test/mjsunit/mjsunit.status

@@ -324,7 +324,6 @@
   'unicodelctest-no-optimization': [PASS, SLOW],
   'unicodelctest': [PASS, SLOW],
   'unicode-test': [PASS, SLOW],
-  'wasm/atomics': [PASS, SLOW],
   'whitespaces': [PASS, SLOW],
 }],  # 'arch == arm64'