Cleanup System.Runtime.Tests

[hughbe]

This is a meta-issue to track progress on cleaning up System.Runtime.Tests. I dumped a massive PR (#5490) which I'm splitting up into smaller ones
The following test files should be cleaned up, to add additional coverage, and follow test guidelines better:

• String (How to change to copy test native binaries in runtest.sh #6296)
• StringBuilder (ldd libcoreclr.so failing on Alpine Linux #6295)
• Number primitives: byte, int, long etc. (Improve LIR dumping in the JIT #7407)
• Floating primitives: float, double (error: pointer being freed was not allocated #7619)
• Decimal ([ARM/Linux] Revisiting FEATURE_FASTTAILCALL issues #6297)
• Array (Environment.set_ExitCode doesn't change process' exit code #6293)
• Array Segment (Installing VS2015Tools-Preview2 from layout broken #6511)
• Buffer ([arm64][1-Failures] Jit\Methodical\cctor\misc\global_il_d\global_il_d.cmd #7614)
• Date primitives: DateTime, DateTimeOffset, TimeSpan (JIT: Eliminate VNConst store-to-load redundancies #6512)
• Char ([Arm64][1-Failure] managed\Compilation\Compilation\Compilation.cmd #7613)
• System.IO exceptions ([x86/Linux] Nested calls don't handle stack alignment properly #7617)
• Activator (Environment.StackTrace includes unnecessary internal calls #6294)
• Attributes (Cannot create list of tuples in assemblies marked as partial trust #7618)
• Enum (Loader.classloader.MethodImpl.self_override test failures #7620)
• System exceptions ([Arm64][2-Failures] String Failures #7616)
• GC (NullReferenceException when using EventListener on linux in some cases #6510)
• Version (ARM64 GCStress: THROWS called in a NOTHROW region #6298)
• Guid (Cannot print exception string because Exception.ToString() failed #6299)
• Nullable ([Arm64] DevDiv_359736 test failures: No Exception thrown #7615)
• Lazy (The .editorconfig should be updated to support the code style options made available in VS2017 #7602)
• Pointers - IntPtr and UIntPtr (Intermittent failures in PAL tests #6300)

After this:

• Add missing namespaces
• Rename tests as per System.Runtime.Tests #15720

[hughbe]

@stephentoub I've sent in 8 PRs for this. I don't want to spam your pull requests page so I'm going to hold off until some of those are reviewed and merged ;)

[jamesqo]

Shouldn't this issue be open? It looks like GitHub may have detected the words 'fix dotnet/corefx#6286' in 652cbda and automatically closed this.

[hughbe]

yup

[hughbe]

Brilliant. I've submitted all the PRs now for this cleanup! Sorry for dumping 10 PRs or so today ;)