base: refactor ImmediateCrashTest.ExpectedOpcodeSequence

This change refactors this test by:
1) Splitting out the library & bytecode-snarfing logic to a helper
   function
2) Deduplicating some code that existed on each platform, via use of
   a couple of typedefs
3) Has the test not hang onto the shared library if one of the gtest
   asserts fail - I don't think this is a problem in practice, but.

Bug: None
Change-Id: Iea33973b141de7cb5fe1aaeae5eb1d6773d31c06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414684Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810407}

base/BUILD.gn

@@ -3219,6 +3219,12 @@ test("base_unittests") {
       "strings/sys_string_conversions_mac_unittest.mm",
     ]
 
+    sources -= [
+      # The immediate crash tests require shared libraries, which we don't
+      # support loading on iOS.
+      "immediate_crash_unittest.cc",
+    ]
+
     if (use_allocator_shim) {
       sources += [
         "allocator/allocator_interception_mac_unittest.mm",