Fix benchmark issues in java/lang/StringIndexOfChar.java benchmark

JDK-8173585 introduced a new benchmark, but it has a few benchmark issues:

 *) Inputs should not be in "static final" to avoid constant folding
 *) Outputs ("ret" values) should be at least returned from @Benchmark method

There might be more issues, please look through the JMH Samples first, and then rewrite.