testsuite: Unset torture_current_flags after use

testsuite: Unset torture_current_flags after use

Before running a test with specific torture options, gcc-dg-runtest
sets the global variable torture_current_flags to the set of torture
options that will be used.  However, it never unset the variable
afterwards, which meant that the last options would hang around
and potentially confuse later non-torture tests.

I saw this with a follow-on patch to check-function-bodies, but it's
probably possible to construct aritificial test combinations that
expose it with check-function-bodies's existing flag filtering.

gcc/testsuite/
	* lib/gcc-dg.exp (gcc-dg-runtest): Unset torture_current_flags
	after each test.