Fix String.init tests with baseline JIT.

Baseline JIT doesn't do deoptimizations, but the test assumes compiled
code will. Adjust the test to call ensureJitCompiled to force
non-baseline compilation.

Test: testrunner.py --host --64 --jit -t 563 -t 575
Bug: 147209001
Change-Id: I062650e07616610b01415e3add193687fcc35e30
2 files changed