Add a WebAssembly filetests directory.

Start adding little 'test compile' test cases which check that the full
compilation pipeline works for each WebAssembly instruction.

filetests/wasm/i32-arith.cton

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+; Test basic code generation for i32 arithmetic WebAssembly instructions.
+test compile
+
+set is_64bit
+isa intel
+
+; Constants.
+
+function %i32_const() -> i32 {
+ebb0:
+    v0 = iconst.i32 0x8765_4321
+    return v0
+}
+
+; Unary operations.
+
+; function %i32_clz(i32) -> i32
+; function %i32_ctz(i32) -> i32
+; function %i32_popcnt(i32) -> i32
+
+; Binary operations.
+
+function %i32_add(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 = iadd v0, v1
+    return v2
+}
+
+function %i32_sub(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 = isub v0, v1
+    return v2
+}
+
+; function %i32_mul(i32, i32) -> i32
+; function %i32_div(i32, i32) -> i32
+; function %i32_rem_s(i32, i32) -> i32
+; function %i32_rem_u(i32, i32) -> i32
+
+function %i32_and(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 = band v0, v1
+    return v2
+}
+
+function %i32_or(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 = bor v0, v1
+    return v2
+}
+
+function %i32_xor(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 = bxor v0, v1
+    return v2
+}
+
+; function %i32_shl(i32, i32) -> i32
+; function %i32_shr_s(i32, i32) -> i32
+; function %i32_shr_u(i32, i32) -> i32
+; function %i32_rotl(i32, i32) -> i32
+; function %i32_rotr(i32, i32) -> i32