[spec/divergence] Add case for nested subshell like ((echo ... (#2399)

gzip has a utility script called zdiff. It gets used at some point in
the check step when building gzip from source. It also makes use of
nested subshells in a few places. The osh parser is tripping over these.
Here's an example from a simple script.

    ((echo foo) | tr o 0)
         ^~~
    _tmp/foo.sh:1: Unexpected token after arithmetic expression (Id.Word_Compound != Id.Arith_RParen)

Other shells seem to handle this case, so in preparation for an inbound
fix, this commit adds a new spec test to cover it.

Author: melvinw

spec/divergence.test.sh

@@ -1,5 +1,5 @@
 ## compare_shells: bash dash mksh zsh ash
-## oils_failures_allowed: 2
+## oils_failures_allowed: 3
 
 # This file relates to:
 #
@@ -55,3 +55,11 @@ bad
 
 ## STDOUT:
 ## END
+
+#### Nested subshell (#2398)
+
+# found on line 137 of the zdiff util from gzip
+((echo foo) | tr o 0)
+## STDOUT:
+f00
+## END