buildtools-tarball: fix default_cases assignment

This is meant to be a list of strings not a single string. For example,
this is overwriting the default in OESDKTestContextExecutor which is:

      default_cases = [os.path.join(os.path.abspath(os.path.dirname(__file__)), 'cases')]

(From OE-Core rev: 867e0a9c571b7bc9a37e6d893f7d75e03fa218a5)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/meta/buildtools-tarball.bb

@@ -135,7 +135,7 @@ python do_testsdk() {
     testsdk = oeqa.sdk.testsdk.TestSDK()
 
     cases_path = os.path.join(os.path.abspath(os.path.dirname(oeqa.sdk.testsdk.__file__)), d.getVar("TESTSDK_CASES"))
-    testsdk.context_executor_class.default_cases = cases_path
+    testsdk.context_executor_class.default_cases = [cases_path,]
 
     testsdk.run(d)
 }