tests: add spdx license header to test unit/link/network/conf files

Those are all consumed by our parser, so they all support comments.
I was considering whether they should have a license header at all,
but in the end I decided to add it because those files are often created
by copying parts of real unit files. And if the real ones have a license,
then those might as well. It's easier to add it than to make an exception.

test/test-execute/exec-ambientcapabilities-merge-nfsnobody.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for AmbientCapabilities
 

test/test-execute/exec-ambientcapabilities-merge-nobody.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for AmbientCapabilities
 

test/test-execute/exec-ambientcapabilities-merge.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for AmbientCapabilities (daemon)
 

test/test-execute/exec-ambientcapabilities-nfsnobody.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for AmbientCapabilities
 

test/test-execute/exec-ambientcapabilities-nobody.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for AmbientCapabilities
 

test/test-execute/exec-ambientcapabilities.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for AmbientCapabilities (daemon)
 

test/test-execute/exec-basic.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for basic execution
 ConditionKernelVersion=">=3.0"

test/test-execute/exec-bindpaths.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for BindPaths= and BindReadOnlyPaths=
 

test/test-execute/exec-capabilityboundingset-invert.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for CapabilityBoundingSet
 

test/test-execute/exec-capabilityboundingset-merge.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for CapabilityBoundingSet
 

test/test-execute/exec-capabilityboundingset-reset.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for CapabilityBoundingSet
 

test/test-execute/exec-capabilityboundingset-simple.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for CapabilityBoundingSet
 

test/test-execute/exec-condition-failed.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for exec condition that fails the unit
 

test/test-execute/exec-condition-skip.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for exec condition that triggers skipping
 

test/test-execute/exec-cpuaffinity1.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for CPUAffinity (simple)
 

test/test-execute/exec-cpuaffinity2.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for CPUAffinity (reset)
 

test/test-execute/exec-cpuaffinity3.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test for CPUAffinity (merge)
 

test/test-execute/exec-dynamicuser-fixeduser-adm.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test DynamicUser with static User= whose uid and gid are different
 # On Fedora, user adm has uid==3 and gid==4.

test/test-execute/exec-dynamicuser-fixeduser-games.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test DynamicUser with static User= whose uid and gid are different
 # On Ubuntu or Debian, user games has uid==5 and gid==60.

test/test-execute/exec-dynamicuser-fixeduser-one-supplementarygroup.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Unit]
 Description=Test DynamicUser with User= and SupplementaryGroups=