Co-authored-by: Vadym Tishchenko <v.tishchenko@evopay.com.ua>

Unset changes len of a `ExpectedCalls` slice during iteration while using index from original slice, and under some conditions it tries to reslice element beyond of the slice boundaries. That causes panic.

The proposed solution uses independent write index to count elements kept in output slice.

Tests (the simplest and more complicated cases) and comments are included.

`MapIndex` returns a zero `reflect.Value` if the map value does not
exist. This now aligns more closely with `InDeltaMapValues` by
checking `reflect/Value.IsValid`.

The use of `recover()` was hiding this error by setting the result of
the test to be "false" despite the test suite passing. This led to
flapping tests where they would succeed if the panic occurred on a
missing key before comparing key values that didn't match!

I've ensured the test suite now asserts on the expected error message
and added another example where the subset has keys not found on the
map under test.