fix(common): add test case and fix smp bugs

Turns out that @darcywong00's suggestion for a test case was a Very Good
Idea. The SMP handling in Keyman Core had an egregious bug where only
half of a surrogate pair was being deleted in the UTF-16 context; see
kmx_processor.cpp 425-428. This was then masked by the test runner
masking the damage in the original test case; see kmx.cpp 193-203.

The new test case exercises a number of additional ways of manipulating
the context where surrogate pairs exist, to show we handle the deletion
properly within Keyman Core and in the test runner.