Convert tst-efgcvt to the new test framework

The function do_test, in tst-efgcvt.c, increments an error counter for
each error that it finds, then returns it to the test framework.
However, the test framework does not expect an error count as return,
but zero for a passing test, one for a failing test, or 77 for an
unsupported test.  Alternatively, the framework provides the function
support_record_failure that records errors, which then allows the test
program to return zero unconditionally.

This patch removes the error counter, replaces each increment of the
counter with a call to support_record_failure, and makes do_test
unconditionally return zero.

Tested for powerpc64le (as-is and with a patched results table to check
that the error reporting actually works).

	* misc/tst-efgcvt.c: Include support/check.h and
	support/test-driver.c.  Do not include test-skeleton.c.
	(error_count): Remove.
	(output_error): Replace increments to error_count with calls to
	support_record_failure.
	(output_r_error): Likewise.
	(special): Likewise.
	(do_test): Unconditionally return zero.
	(TEST_FUNCTION): Remove.