Use "test regalloc" for the register allocator tests.

These tests were only using "test compile" because it doesn't require
any filecheck directives to be present, so just stop requiring filecheck
directives for "test regalloc" and other filecheck-based test drivers.

cranelift/filetests/regalloc/output-interference.cton

@@ -1,4 +1,4 @@
-test compile
+test regalloc
 set is_64bit=1
 isa intel haswell