libsframe: testsuite: remove usage of ##__VA_ARGS__

libsframe: testsuite: remove usage of ##__VA_ARGS__

'##__VA_ARGS' is a GNU extension, the usage of which in the testsuite of
libsframe may cause failure to build on platforms where the compiler may
not support the GNU extension.

This GNU extension swallows the preceding comma if the variable
arguments list is empty.  In libsframe testsuite though, an empty list
is never used.  Usages will remain of the following type:
  - TEST (cond, "string", vars);
or
  - TEST (cond, "string");

Mailing list discussion:
https://sourceware.org/pipermail/binutils/2025-November/145825.html

PR libsframe/33437

libsframe/testsuite/
	PR libsframe/33437
	* sframe-test.h: Replace ##__VA_ARGS__ with __VA_ARGS__.