[mlir][spirv] Rename *.spv tests to *.spvasm. (llvm#170161)

This patch renames two of the SPIR-V tests to `*.spvasm` since both
files are assembly files, rather than SPIR-V binaries. The `lit.cfg.py`
is adjusted and we no longer need to run `*.spv` tests since none are
present.

Author: IgWod-IMG

mlir/test/Target/SPIRV/consecutive-selection.spv renamed to mlir/test/Target/SPIRV/consecutive-selection.spvasm

@@ -44,7 +44,7 @@
     ".test",
     ".pdll",
     ".c",
-    ".spv",
+    ".spvasm",
 ]
 
 # test_source_root: The root path where tests are located.