testsuite: Run vec_insert case on P8 and P9 with option specified

testsuite: Run vec_insert case on P8 and P9 with option specified

Move run_test and TEST_VEC_INSERT_ALL to header file for share usage.

gcc/testsuite/ChangeLog:

2021-01-29  Xionghu Luo  <luoxhu@linux.ibm.com>

	* gcc.target/powerpc/pr79251.p8.c: Move TEST_VEC_INSERT_ALL
	to ...
	* gcc.target/powerpc/pr79251.h: ...this.
	* gcc.target/powerpc/pr79251.p9.c: Likewise.
	* gcc.target/powerpc/pr79251-run.c: Move run_test to pr79251.h.
	Rename to...
	* gcc.target/powerpc/pr79251-run.p8.c: ...this.
	* gcc.target/powerpc/pr79251-run.p9.c: New test.