fix(tests): eliminate UtcNow race in GetChainCertificateTargetStore_NonSelfSigned test

The non-self-signed chain-routing test called DateTimeOffset.UtcNow.AddDays(1)
twice -- once for the root cert and once for the intermediate cert -- so when
the wall clock ticked a second between the two calls the intermediate's
notAfter ended up later than the root's notAfter, and CertificateRequest.Create
rejected it:

  System.ArgumentException : The requested notAfter value (...:11) is later
  than issuerCertificate.NotAfter (...:10). (Parameter 'notAfter')

Capture notBefore/notAfter once at the top of the test and reuse the same
DateTimeOffset for both certificates so the intermediate's validity window
is guaranteed equal to (not later than) the issuer's.

TESTS
- 216/216 passing locally; this was reliably reproducible under CI load
  (latest failure on commit ceea76255b).

src/PSInfisicalAPI.Tests/CsrAndRequestCmdletTests.cs

@@ -409,6 +409,9 @@ namespace PSInfisicalAPI.Tests
             using (System.Security.Cryptography.RSA rootRsa = System.Security.Cryptography.RSA.Create(2048))
             using (System.Security.Cryptography.RSA intermediateRsa = System.Security.Cryptography.RSA.Create(2048))
             {
+                DateTimeOffset notBefore = DateTimeOffset.UtcNow.AddMinutes(-5);
+                DateTimeOffset notAfter  = DateTimeOffset.UtcNow.AddDays(1);
+
                 System.Security.Cryptography.X509Certificates.CertificateRequest rootRequest = new System.Security.Cryptography.X509Certificates.CertificateRequest(
                     "CN=ChainRouting.Root",
                     rootRsa,
@@ -416,7 +419,7 @@ namespace PSInfisicalAPI.Tests
                     System.Security.Cryptography.RSASignaturePadding.Pkcs1);
                 rootRequest.CertificateExtensions.Add(new System.Security.Cryptography.X509Certificates.X509BasicConstraintsExtension(true, false, 0, true));
 
-                using (System.Security.Cryptography.X509Certificates.X509Certificate2 rootCert = rootRequest.CreateSelfSigned(DateTimeOffset.UtcNow.AddMinutes(-5), DateTimeOffset.UtcNow.AddDays(1)))
+                using (System.Security.Cryptography.X509Certificates.X509Certificate2 rootCert = rootRequest.CreateSelfSigned(notBefore, notAfter))
                 {
                     System.Security.Cryptography.X509Certificates.CertificateRequest intermediateRequest = new System.Security.Cryptography.X509Certificates.CertificateRequest(
                         "CN=ChainRouting.Intermediate",
@@ -426,7 +429,7 @@ namespace PSInfisicalAPI.Tests
                     intermediateRequest.CertificateExtensions.Add(new System.Security.Cryptography.X509Certificates.X509BasicConstraintsExtension(true, false, 0, true));
 
                     byte[] serial = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 };
-                    using (System.Security.Cryptography.X509Certificates.X509Certificate2 intermediate = intermediateRequest.Create(rootCert, DateTimeOffset.UtcNow.AddMinutes(-5), DateTimeOffset.UtcNow.AddDays(1), serial))
+                    using (System.Security.Cryptography.X509Certificates.X509Certificate2 intermediate = intermediateRequest.Create(rootCert, notBefore, notAfter, serial))
                     {
                         Assert.NotEqual(intermediate.Subject, intermediate.Issuer);