coroutines, testsuite: Tidy some test names (NFC).

Try to make sure that we have consistent naming for the tests
before first release.

gcc/testsuite/ChangeLog:

2020-04-22 Iain Sandoe <iain@sandoe.co.uk>

	* g++.dg/coroutines/torture/local-var-0.C: Rename to...
	* g++.dg/coroutines/torture/local-var-00-const.C: ...this.
	* g++.dg/coroutines/torture/local-var-1.C: Rename to...
	* g++.dg/coroutines/torture/local-var-01-single.C: ...this.
	* g++.dg/coroutines/torture/local-var-2.C: Rename to...
	* g++.dg/coroutines/torture/local-var-02-conditional.C: ...this.
	* g++.dg/coroutines/torture/local-var-3.C: Rename to...
	* g++.dg/coroutines/torture/local-var-03-with-awaits.C: ...this.
	* g++.dg/coroutines/torture/local-var-4.C: Rename to...
	* g++.dg/coroutines/torture/local-var-04-hiding-nested-scopes.C: this.
	* g++.dg/coroutines/torture/local-var-5-awaitable.C: Rename to...
	* g++.dg/coroutines/torture/local-var-05-awaitable.C: ...this.