profopt-execute: unset testname_with_flags if create_gcov fails

When diffing test results, there sometimes occur spurious "New tests
that PASS" / "Old tests that passed, that have disappeared" messages.
The reason is that if create_gcov is not installed, then the cached
testname_with_flags is not cleared and is carried over to the next
test.

gcc/testsuite/ChangeLog:

2020-11-26  Ilya Leoshkevich  <iii@linux.ibm.com>

	* lib/profopt.exp: Unset testname_with_flags if create_gcov
	fails.

gcc/testsuite/lib/profopt.exp

@@ -456,6 +456,7 @@ proc profopt-execute { src } {
 		set id [remote_spawn "" $cmd]
 		if { $id < 0 } {
 		    unsupported "$testcase -fauto-profile: cannot run create_gcov"
+		    unset testname_with_flags
 		    set status "fail"
 		    return
 		}