Add tests to increase code coverage for Span<T> APIs

[ahsonkhan]

Here is a snapshot of the coverage:

Currently, SpanHelpers, specifically SpanHelpers.Clear.cs needs tests.
https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/System.Memory_SpanHelpers.htm

[ahsonkhan]

cc @KrzysztofCwalina, @shiftylogic
Why is the CI showing low (zero) test coverage for Span.Clear?
Specifically, here it shows 0 code coverage. https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/System.Memory_SpanHelpers.htm#Djworkspacecode_coverageb14a30e5srcSystemMemorysrcSystemSpanHelpersClearcs

But we have extensive Span.Clear tests (which get discovered and run/pass locally):
https://github.com/dotnet/corefx/blob/master/src/System.Memory/tests/Span/Clear.cs#L9

The tests also run in CI:
https://ci.dot.net/job/dotnet_corefx/job/master/job/windows_nt_release_prtest/5033/testReport/System.SpanTests/SpanTests/

[shiftylogic]

Until yesterday, I'm pretty sure those tests were never running against slow Span. I added netfx to the test configuration, but we might need to add more.

[shiftylogic]

FYI, if you run code coverage locally against System.Memory using the netfx framework, the numbers look much better.

Locally (not netfx):

[ahsonkhan]

Code Coverage

SlowSpan:

FastSpan:

[ahsonkhan]

System.Private.Corelib Code Coverage (for Span):