cranelift: Merge interpreter tests with runtests (#3252)

Almost all the tests in the interpreter are already in the runtests
folder so that we can reuse them for the backends. The distinction
between interpreter tests and runtests is no longer very clear, since
they should both support the same clif code, and produce the same results.

We only have two test files:

* `add.clif` tests the add and jump instruction, both of which are
already covered in other test files, so we remove that file.

* `fibonacci.clif` does a recursive call which is currently not supported
in the filetest environment, so we keep this test interpreter only for now.

cranelift/filetests/filetests/interpreter/add.clif

@@ -1,13 +0,0 @@
-test interpret
-
-function %add() -> i32 {
-block0:
-    v0 = iconst.i32 40
-    v1 = iconst.i32 2
-    v2 = iadd v0, v1
-    jump block1(v2)
-
-block1(v3: i32):
-    return v3
-}
-; run: %add() == 42