Test | Convert SqlVariantParameterTests to xunit assertions (dotnet#4034)

* Move test logic from SqlVariantParam to SqlVariantParameterTests

* Convert SqlVariantParameterTests to xunit assertions

* Remove baseline files and parsing logic

* Fix SqlString TVP collation issue on Linux

The default SqlString constructor uses CultureInfo.CurrentCulture.LCID,
which on Linux returns 127 (InvariantCulture). LCID 127 is not a valid
SQL Server collation. The TVP code path (ValueUtilsSmi.SetSqlString_Unchecked)
encodes the SqlString's LCID directly into the TDS stream, unlike the regular
parameter path which uses the server's default collation.

Use explicit LCID 1033 (en-US) with matching compare options to ensure a
valid SQL Server collation is always used in the test.

* Remove globalization using.

* Work around SqlString LCID product bug in TVP test

ValueUtilsSmi.GetSqlValue reconstructs SqlString values from Variant columns
using new SqlString(string), which uses CultureInfo.CurrentCulture.LCID.
On Linux, this is 127 (InvariantCulture) - not a valid SQL Server collation.
The original test worked around this by setting Thread.CurrentThread.CurrentCulture
to en-US before running. Restore that pattern with a clear comment explaining
why it's needed.

Revert SqlString test data back to the simple constructor since setting an
explicit LCID on it has no effect - the LCID is lost when the value is read
back from SqlDataRecord during TVP serialization.

* Fix collation. Split tests down even further to aid debugging.

* Address copilot comments.

* Clarify special bulk copy behavior

* Improve readability.

* Remove baseline file missed in conflicts with main.

* Disable discovery enumeration of tests.

Author: mdaigle

src/Microsoft.Data.SqlClient/tests/ManualTests/Microsoft.Data.SqlClient.ManualTests.csproj

@@ -213,7 +213,6 @@
     <Compile Include="SQL\ParameterTest\DateTimeVariantTest.cs" />
     <Compile Include="SQL\ParameterTest\ParametersTest.cs" />
     <Compile Include="SQL\ParameterTest\SqlAdapterUpdateBatch.cs" />
-    <Compile Include="SQL\ParameterTest\SqlVariantParam.cs" />
     <Compile Include="SQL\ParameterTest\SteAttribute.cs" />
     <Compile Include="SQL\ParameterTest\SteParam.cs" />
     <Compile Include="SQL\ParameterTest\StePermutationSet.cs" />
@@ -269,10 +268,6 @@
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Link>DateTimeVariant.bsl</Link>
     </Content>
-    <Content Include="SQL\ParameterTest\SqlVariantParameter.bsl">
-      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
-      <Link>SqlVariantParameter.bsl</Link>
-    </Content>
     <Content Include="SQL\ParameterTest\StreamInputParameter_DebugMode.bsl">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Link>StreamInputParameter_DebugMode.bsl</Link>