Exclude Migrations namespace from dotnet code coverage

EF migrations are generated scaffolding with no meaningful logic to
test; counting them against the coverage target only dilutes the
signal.

tests/api/MicCheck.Api.Tests.Unit/coverlet.runsettings

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<RunSettings>
+  <DataCollectionRunSettings>
+    <DataCollectors>
+      <DataCollector friendlyName="XPlat code coverage">
+        <Configuration>
+          <Exclude>[MicCheck.Api]MicCheck.Api.Migrations.*</Exclude>
+        </Configuration>
+      </DataCollector>
+    </DataCollectors>
+  </DataCollectionRunSettings>
+</RunSettings>