target-xtensa: add overridable test_init macro

Some test suites, like MMU, need per-test initialization. Don't make them
redefine test macro, add test_init for that purpose.
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>

tests/tcg/xtensa/macros.inc

@@ -43,8 +43,12 @@ main:
     simcall
 .endm
 
+.macro test_init
+.endm
+
 .macro test name
     //print test_\name
+    test_init
 test_\name:
 .global test_\name
 .endm

tests/tcg/xtensa/test_mmu.S

@@ -2,9 +2,9 @@
 
 test_suite mmu
 
-.purgem test
+.purgem test_init
 
-.macro test name
+.macro test_init
     movi    a2, 0x00000004
     idtlb   a2
     movi    a2, 0x00100004