tests: add allocation failure test for buffers

Test that `git_buf` correctly fails if no more bytes can be allocated.
This is mostly for demonstration purposes.

tests/util/str/oom.c

 #include "clar_libgit2.h"
+#include "clar_libgit2_alloc.h"
 
 /* Override default allocators with ones that will fail predictably. */
 
@@ -56,3 +57,15 @@ void test_str_oom__grow_by(void)
 	cl_assert(git_str_grow_by(&buf, 101) == -1);
 	cl_assert(git_str_oom(&buf));
 }
+
+void test_str_oom__allocation_failure(void)
+{
+	git_str buf = GIT_STR_INIT;
+
+	cl_alloc_limit(10);
+
+	cl_git_pass(git_str_puts(&buf, "foobar"));
+	cl_git_fail(git_str_puts(&buf, "foobar"));
+
+	cl_alloc_reset();
+}