[vm] Refactor vm/dart/use_dwarf_stack_traces_flag tests.

Both the regular and deferred versions of this test contain
up to six different test cases:

All platforms:
 * ELF snapshot, using DWARF from the snapshot
 * ELF snapshot, using DWARF from the separate debugging information
For platforms where the test can assemble snapshots:
 * assembled snapshot, using DWARF from the snapshot or
   the separate .dSYM package on MacOS
 * assembled snapshot, using DWARF from the separate debugging
   information
For MacOS only:
 * creating a single-architecture universal binary from the
   separate .dSYM package and extracting DWARF information from it
 * creating a multi-architecture universal binary from the
   separate .dSYM package and extracting DWARF information from it

Originally the tests were written using package:expect, performing
program compilation and execution separately before each test and
lazily reading DWARF information within the test itself. Since tests
using package:expect stop the program immediately on a failure,
a failing expectation keeps other independent test cases from being
checked. However, it's useful to know if the failure is limited to
only a subset of the test cases, since that helps point at which code
is to blame for the test failure(s).

Now the tests are refactored to first set up the tests by performing
all program compilation and execution first, collecting all outputs and
DWARF information as test state. Then the test cases are defined over
the collected test state using package:test instead of package:expect.
This way, as long as there is not a failure in the initial setup, all
of the applicable test cases are run even if one or more of them fail.

-----

In pkg/native_stack_traces, changes the return types of the reader
retrieval methods in the DwarfContainer class and
DwarfSnapshot.fromDwarfContainer to be nullable. If the shared object
does not contain the expected DWARF information, these methods now
return null instead of causing null check exceptions to be thrown.

Issue: https://github.com/dart-lang/sdk/issues/55612

Change-Id: I253965a95894f455e51d021e32dbf2703d8b99cf
Cq-Include-Trybots: luci.dart.try:vm-aot-dwarf-linux-product-x64-try,vm-aot-linux-debug-x64-try,vm-aot-mac-release-arm64-try,vm-aot-mac-product-arm64-try,vm-aot-mac-release-x64-try,vm-aot-win-debug-arm64-try,vm-aot-win-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/375240
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>