Fix duplicate test runs on Windows

[atifaziz]

This PR fixes test.cmd that was running same test configuration several times since the framework-specific flag was missing.

[atifaziz]

For example, just a test run for netcoreapp2.1 (Debug) would also test the netcoreapp3.1 target:

Testing netcoreapp2.1 (Debug)...
Test run for C:\projects\morelinq\MoreLinq.Test\bin\Debug\netcoreapp3.1\MoreLinq.Test.dll (.NETCoreApp,Version=v3.1)
Microsoft (R) Test Execution Command Line Tool Version 16.8.0
Copyright (c) Microsoft Corporation.  All rights reserved.
Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
  Skipped MemoizeIsThreadSafe [< 1 ms]
  Skipped PartialSortByIsStable [< 1 ms]
  Skipped PartialSortByIsStable [< 1 ms]
  Skipped TestRandomSubsetIsUnbiased [< 1 ms]
Passed!  - Failed:     0, Passed:  2200, Skipped:     2, Total:  2202, Duration: 3 s - MoreLinq.Test.dll (netcoreapp3.1)
Calculating coverage result...
  Generating report 'C:\projects\morelinq\MoreLinq.Test\coverage.netcoreapp3.1.opencover.xml'
+--------+------+--------+--------+
| Module | Line | Branch | Method |
+--------+------+--------+--------+
+---------+------+--------+--------+
|         | Line | Branch | Method |
+---------+------+--------+--------+
| Total   | 100% | 100%   | 100%   |
+---------+------+--------+--------+
| Average | NaN% | NaN%   | NaN%   |
+---------+------+--------+--------+
Test run for C:\projects\morelinq\MoreLinq.Test\bin\Debug\netcoreapp2.1\MoreLinq.Test.dll (.NETCoreApp,Version=v2.1)
Microsoft (R) Test Execution Command Line Tool Version 16.8.0
Copyright (c) Microsoft Corporation.  All rights reserved.
Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
  Skipped MemoizeIsThreadSafe [1 ms]
  Skipped PartialSortByIsStable
  Skipped PartialSortByIsStable
  Skipped TestRandomSubsetIsUnbiased
Passed!  - Failed:     0, Passed:  2200, Skipped:     2, Total:  2202, Duration: 3 s - MoreLinq.Test.dll (netcoreapp2.1)
Calculating coverage result...
  Generating report 'C:\projects\morelinq\MoreLinq.Test\coverage.netcoreapp2.1.opencover.xml'
+--------+------+--------+--------+
| Module | Line | Branch | Method |
+--------+------+--------+--------+
+---------+------+--------+--------+
|         | Line | Branch | Method |
+---------+------+--------+--------+
| Total   | 100% | 100%   | 100%   |
+---------+------+--------+--------+
| Average | NaN% | NaN%   | NaN%   |
+---------+------+--------+--------+
Test run for C:\projects\morelinq\MoreLinq.Test\bin\Debug\net451\MoreLinq.Test.exe (.NETFramework,Version=v4.5.1)
Microsoft (R) Test Execution Command Line Tool Version 16.8.0
Copyright (c) Microsoft Corporation.  All rights reserved.
Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
  Skipped MemoizeIsThreadSafe [1 ms]
  Skipped PartialSortByIsStable
  Skipped PartialSortByIsStable
  Skipped TestRandomSubsetIsUnbiased
Passed!  - Failed:     0, Passed:  2200, Skipped:     2, Total:  2202, Duration: 6 s - MoreLinq.Test.exe (net451)
Calculating coverage result...
  Generating report 'C:\projects\morelinq\MoreLinq.Test\coverage.net451.opencover.xml'
+--------+------+--------+--------+
| Module | Line | Branch | Method |
+--------+------+--------+--------+
+---------+------+--------+--------+
|         | Line | Branch | Method |
+---------+------+--------+--------+
| Total   | 100% | 100%   | 100%   |
+---------+------+--------+--------+
| Average | NaN% | NaN%   | NaN%   |
+---------+------+--------+--------+

This would then repeat when the intention was to test other targets individually.

[codecov]

Codecov Report

Merging #769 (31d6709) into master (9dd2d5e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #769   +/-   ##
=======================================
  Coverage   87.90%   87.90%           
=======================================
  Files         108      108           
  Lines        3473     3473           
  Branches      984      984           
=======================================
  Hits         3053     3053           
  Misses        272      272           
  Partials      148      148           

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