serverutils: move an assertion to a better place

Prior to this patch, the assertion on the value of DefaultTestTenant
was inside `ShouldStartDefaultTestTenant()`. However, this function
is only ever called from `StartServer`; it is not called from
`NewServer()`. This means that tests that call `NewServer` directly do
not get the benefit of the validity check.

This patch moves the assertion to the better place.

Release note: None

pkg/testutils/serverutils/test_server_shim.go

@@ -75,12 +75,6 @@ var PreventStartTenantError = errors.New("attempting to manually start a virtual
 func ShouldStartDefaultTestTenant(
 	t TestLogger, baseArg base.DefaultTestTenantOptions,
 ) (retval base.DefaultTestTenantOptions) {
-	defer func() {
-		if !(retval.TestTenantAlwaysEnabled() || retval.TestTenantAlwaysDisabled()) {
-			panic(errors.AssertionFailedf("programming error: no decision was actually taken"))
-		}
-	}()
-
 	// Explicit cases for enabling or disabling the default test tenant.
 	if baseArg.TestTenantAlwaysEnabled() {
 		return baseArg
@@ -243,6 +237,10 @@ func NewServer(params base.TestServerArgs) (TestServerInterface, error) {
 		return nil, errors.AssertionFailedf("TestServerFactory not initialized. One needs to be injected " +
 			"from the package's TestMain()")
 	}
+	tcfg := params.DefaultTestTenant
+	if !(tcfg.TestTenantAlwaysEnabled() || tcfg.TestTenantAlwaysDisabled()) {
+		return nil, errors.AssertionFailedf("programming error: DefaultTestTenant does not contain a decision\n(maybe call ShouldStartDefaultTestTennat?)")
+	}
 
 	srv, err := srvFactoryImpl.New(params)
 	if err != nil {