[tests/nnbd] Fix tests/language/lazy/static8_test

This test exercises recursive static field initialization.
Under the new language semantics all static fields are implicitly late,
The test was using 'final' fields, which can be initialized only once,
but the test attempted to initialize those fields multiple times
during recursive invocation. This results in LateInitializationError.

So the test is updated to use non-final fields which allow
multiple writes.

Relevant paragraph from the spec:

Let D be a late and final local variable declaration named v. It is a run-time error, throwing an instance of LateInitializationError, to assign a value to v if a value has previously been assigned to v.
Note that this includes the implicit initializing writes induced by evaluating the initializer during a read.

Change-Id: I577cb9d189c1e6975ea5f21fab64d5b1204315d8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154823
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>