Tests for String.setCharAt() breaking string compression.

With string compression, all compressible strings must be
compressed. The internal API String.setCharAt() can break
that invariant when overwriting a non-ASCII character with
an ASCII character, turning an uncompressible string into
a compressible one. It can also truncate a non-ASCII
character written to a compressed string. These regression
tests check the public API that exposes the problem.
Submitting these tests (without a fix) shall prevent us
from enabling string compression before it's ready.

Test: testrunner.py --host -t 021-string2
Test: Manually check that new asserts fail with string compression.
Bug: 31040547
Change-Id: I66f27a73f273f7648acbdf1b601345711f37c85e