Added license headers

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>

dsp_ff-plugin/tests/nexus_conn_conflict/nexus_conn_conflict.v

diff --git a/dsp_ff-plugin/tests/nexus_conn_conflict/nexus_conn_conflict.v b/dsp_ff-plugin/tests/nexus_conn_conflict/nexus_conn_conflict.v
index 2eb8634..2224690 100644
--- a/dsp_ff-plugin/tests/nexus_conn_conflict/nexus_conn_conflict.v
+++ b/dsp_ff-plugin/tests/nexus_conn_conflict/nexus_conn_conflict.v
@@ -1,3 +1,11 @@
+// Copyright (C) 2020-2021  The SymbiFlow Authors.
+//
+// Use of this source code is governed by a ISC-style
+// license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/ISC
+//
+// SPDX-License-Identifier:ISC
+
 module conflict_dsp_clk (
     input  wire        CLK_A,
     input  wire        CLK_B,

dsp_ff-plugin/tests/nexus_conn_share/nexus_conn_share.v

diff --git a/dsp_ff-plugin/tests/nexus_conn_share/nexus_conn_share.v b/dsp_ff-plugin/tests/nexus_conn_share/nexus_conn_share.v
index 856cda8..88be6b0 100644
--- a/dsp_ff-plugin/tests/nexus_conn_share/nexus_conn_share.v
+++ b/dsp_ff-plugin/tests/nexus_conn_share/nexus_conn_share.v
@@ -1,3 +1,11 @@
+// Copyright (C) 2020-2021  The SymbiFlow Authors.
+//
+// Use of this source code is governed by a ISC-style
+// license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/ISC
+//
+// SPDX-License-Identifier:ISC
+
 module conflict_out_fanout (
     input  wire        CLK,
     input  wire [ 8:0] A,

dsp_ff-plugin/tests/nexus_fftypes/nexus_fftypes.v

diff --git a/dsp_ff-plugin/tests/nexus_fftypes/nexus_fftypes.v b/dsp_ff-plugin/tests/nexus_fftypes/nexus_fftypes.v
index e4f22fe..2c5b8ec 100644
--- a/dsp_ff-plugin/tests/nexus_fftypes/nexus_fftypes.v
+++ b/dsp_ff-plugin/tests/nexus_fftypes/nexus_fftypes.v
@@ -1,3 +1,11 @@
+// Copyright (C) 2020-2021  The SymbiFlow Authors.
+//
+// Use of this source code is governed by a ISC-style
+// license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/ISC
+//
+// SPDX-License-Identifier:ISC
+
 module mult_ena (
     input  wire        CLK,
     input  wire        ENA,

dsp_ff-plugin/tests/nexus_mult/nexus_mult.v

diff --git a/dsp_ff-plugin/tests/nexus_mult/nexus_mult.v b/dsp_ff-plugin/tests/nexus_mult/nexus_mult.v
index 4b7bda4..1ca806d 100644
--- a/dsp_ff-plugin/tests/nexus_mult/nexus_mult.v
+++ b/dsp_ff-plugin/tests/nexus_mult/nexus_mult.v
@@ -1,3 +1,11 @@
+// Copyright (C) 2020-2021  The SymbiFlow Authors.
+//
+// Use of this source code is governed by a ISC-style
+// license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/ISC
+//
+// SPDX-License-Identifier:ISC
+
 module mult_ireg (
     input  wire        CLK,
     input  wire [ 8:0] A,

dsp_ff-plugin/tests/nexus_param_conflict/nexus_param_conflict.v

diff --git a/dsp_ff-plugin/tests/nexus_param_conflict/nexus_param_conflict.v b/dsp_ff-plugin/tests/nexus_param_conflict/nexus_param_conflict.v
index d03d611..d9ec0e8 100644
--- a/dsp_ff-plugin/tests/nexus_param_conflict/nexus_param_conflict.v
+++ b/dsp_ff-plugin/tests/nexus_param_conflict/nexus_param_conflict.v
@@ -1,3 +1,11 @@
+// Copyright (C) 2020-2021  The SymbiFlow Authors.
+//
+// Use of this source code is governed by a ISC-style
+// license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/ISC
+//
+// SPDX-License-Identifier:ISC
+
 module conflict_dsp_ctrl_param (
     input  wire        CLK,
     input  wire [ 8:0] A,