Tests: Add case for rejecting $<TARGET_FILE:...> on an INTERFACE library

Add an explicit test for this existing error.  We already cover it for
an OBJECT library but INTERFACE libraries are not allowed either.

Tests/RunCMake/GeneratorExpression/BadTargetTypeInterface-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/GeneratorExpression/BadTargetTypeInterface-stderr.txt

@@ -0,0 +1,26 @@
+CMake Error at BadTargetTypeInterface.cmake:[0-9]+ \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<TARGET_FILE:iface>
+
+  Target "iface" is not an executable or library.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)
++
+CMake Error at BadTargetTypeInterface.cmake:[0-9]+ \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<TARGET_SONAME_FILE:iface>
+
+  Target "iface" is not an executable or library.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)
++
+CMake Error at BadTargetTypeInterface.cmake:[0-9]+ \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<TARGET_LINKER_FILE:iface>
+
+  Target "iface" is not an executable or library.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)

Tests/RunCMake/GeneratorExpression/BadTargetTypeInterface.cmake

@@ -0,0 +1,6 @@
+add_library(iface INTERFACE)
+add_custom_target(check ALL COMMAND echo
+  $<TARGET_FILE:iface>
+  $<TARGET_SONAME_FILE:iface>
+  $<TARGET_LINKER_FILE:iface>
+  )

Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake

@@ -7,6 +7,7 @@ run_cmake(BadNOT)
 run_cmake(BadStrEqual)
 run_cmake(BadZero)
 run_cmake(BadTargetName)
+run_cmake(BadTargetTypeInterface)
 run_cmake(BadTargetTypeObject)
 run_cmake(BadInstallPrefix)
 run_cmake(CMP0044-WARN)