pw_allocator: Check for room to split the first block

Changing the check in Block::AllocLast(). Usually, during an
allocation, a free block is checked for enough space for the allocation.
If there more than enough space, but not enough to hold another block,
the excess is appended to the previous Block. This is not possible with
the first Block since there isn't a previous Block. In the case of the
first Block, the extra space will become part of the block and the Block
will be returned to the caller. Before this change, even when there was
not enough room for the first Block to be split into two, it was. This
caused heap corruption and eventually a crash.

As part of this fix, the unit tests were made less brittle and more
exhaustive by adding test utilities that manage aligned and unaligned
buffers, and then ensuring unit test coverage of every path though
AllocFirst, CanAllocLast, and AllocLast.

Bug: 366175024
Change-Id: Ic3c9538e5e33eaf507f08f0d3ac54361b5dcf998
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/235312
Pigweed-Auto-Submit: Aaron Green <[email protected]>
Reviewed-by: John Comito <[email protected]>
Reviewed-by: Ewout van Bekkum <[email protected]>
Lint: Lint 🤖 <[email protected]>
Reviewed-by: Keir Mierle <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Docs-Not-Needed: Aaron Green <[email protected]>
Presubmit-Verified: CQ Bot Account <[email protected]>

pw_allocator/BUILD.bazel

@@ -625,6 +625,7 @@ pw_cc_test(
         "freelist_heap_test.cc",
     ],
     deps = [
+        ":block_testing",
         ":freelist_heap",
         "//pw_bytes:alignment",
     ],

pw_allocator/BUILD.gn

@@ -486,6 +486,7 @@ pw_test("fragmentation_test") {
 
 pw_test("freelist_heap_test") {
   deps = [
+    ":block_testing",
     ":freelist_heap",
     "$dir_pw_bytes:alignment",
   ]

pw_allocator/CMakeLists.txt

@@ -573,6 +573,7 @@ pw_add_test(pw_allocator.freelist_heap_test
   SOURCES
     freelist_heap_test.cc
   PRIVATE_DEPS
+    pw_allocator.block_testing
     pw_allocator.freelist_heap
     pw_bytes.alignment
   GROUPS