[gdb/testsuite] Handle maint set dwarf synchronous off default

I ran the testsuite with a patch setting dwarf_synchronous to false by
default, and ran into FAILs in test-cases gdb.dwarf2/dw2-inter-cu-error.exp
and gdb.dwarf2/dw2-inter-cu-error-2.exp, because the expected DWARF errors did
not show up as a result of the file command.

Fix this by forcing "maint set dwarf synchronous on".

Add the same in gdb.base/index-cache.exp, where this is also required.

Tested on aarch64-linux.

gdb/testsuite/gdb.base/index-cache.exp

@@ -86,7 +86,11 @@ proc run_test_with_flags { cache_dir cache_enabled code } {
 	set GDBFLAGS "$GDBFLAGS -iex \"set index-cache directory $cache_dir\""
 	set GDBFLAGS "$GDBFLAGS -iex \"set index-cache enabled $cache_enabled\""
 
-	clean_restart ${testfile}
+	clean_restart
+
+	gdb_test_no_output "maint set dwarf synchronous on"
+
+	gdb_load $::binfile
 
 	uplevel 1 $code
     }

gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error-2.exp

@@ -41,11 +41,17 @@ Dwarf::assemble $asm_file {
     }
 }
 
-if [prepare_for_testing "failed to prepare" $testfile \
+if [build_executable "failed to prepare" $testfile \
 	[list $asm_file $srcfile] {nodebug}] {
     return -1
 }
 
+clean_restart
+
+gdb_test_no_output "maint set dwarf synchronous on"
+
+gdb_load $binfile
+
 gdb_assert \
     { [regexp "DWARF Error: cannot follow reference" $gdb_file_cmd_msg] } \
     "Error message"

gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error.exp

@@ -41,11 +41,17 @@ Dwarf::assemble $asm_file {
     }
 }
 
-if [prepare_for_testing "failed to prepare" $testfile \
+if [build_executable "failed to prepare" $testfile \
 	[list $asm_file $srcfile] {nodebug}] {
     return -1
 }
 
+clean_restart
+
+gdb_test_no_output "maint set dwarf synchronous on"
+
+gdb_load $binfile
+
 # Verify that GDB notices the null DIE.
 gdb_assert { [regexp "DWARF Error: Unexpected null DIE" $gdb_file_cmd_msg] } \
 	"Null DIE error missing"