[gn] Add dummy build file for SPIRVTests

sync_source_lists_from_cmake.py checks that every unittest in CMake
also exists in the GN build. 7b08b43 added SPRIVTests, but the GN
build doesn't include the SPIRV target. So add a dummy target for this
to placate the check.

See also VETests in the gn build, which has the same setup.

llvm/utils/gn/secondary/llvm/unittests/Target/SPIRV/BUILD.gn

@@ -0,0 +1,4 @@
+# FIXME: If we add the SPIRV target to the GN build, add SPRIVTests here.
+group("dummy") {
+  sources = [ "SPIRVConvergenceRegionAnalysisTests.cpp" ]
+}