hghave: add a check for unzip(1) that understands symlinks

unzip(1) from the FreeBSD base system does not understand symlinks, so
test-archive-symlinks is busted.

tests/hghave.py

@@ -573,3 +573,7 b' def has_hypothesis():'
         return True
     except ImportError:
         return False
+
+@check("unziplinks", "unzip(1) understands and extracts symlinks")
+def unzip_understands_symlinks():
+    return matchoutput('unzip --help', br'Info-ZIP')