classes/testimage: Fix exportTests function.

With new structure of TestContext now holds suite and variable
that contains unittest instances,  it can't be exported using
JSON causing and exception.

Adds the suite variable for avoid export it.

(From OE-Core rev: cecc7ec2bcb28d7d8a3277fb097efbbe13adff1c)

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/testimage.bbclass

@@ -112,7 +112,7 @@ def exportTests(d,tc):
     savedata["host_dumper"] = {}
     for key in tc.__dict__:
         # special cases
-        if key != "d" and key != "target" and key != "host_dumper":
+        if key not in ['d', 'target', 'host_dumper', 'suite']:
             savedata[key] = getattr(tc, key)
     savedata["target"]["ip"] = tc.target.ip or d.getVar("TEST_TARGET_IP", True)
     savedata["target"]["server_ip"] = tc.target.server_ip or d.getVar("TEST_SERVER_IP", True)