Minor "require" fixups

I found a couple of spots that could use "require", and one spot where
hoisting the "require" closer to the top of the file made it more
clear.

gdb/testsuite/gdb.btrace/enable.exp

diff --git a/gdb/testsuite/gdb.btrace/enable.exp b/gdb/testsuite/gdb.btrace/enable.exp
index 6a30d7d..bedff46 100644
--- a/gdb/testsuite/gdb.btrace/enable.exp
+++ b/gdb/testsuite/gdb.btrace/enable.exp
@@ -76,10 +76,7 @@
 # skip the rerun test when using gdbserver
 # otherwise rerun twice, target should be automatically disabled
 load_lib gdbserver-support.exp
-if {![allow_gdbserver_tests]} {
-    unsupported "target does not support gdbserver"
-    return 0
-}
+require allow_gdbserver_tests
 clean_restart $testfile
 if ![runto_main] {
     return -1

gdb/testsuite/gdb.mi/list-thread-groups-available.exp

diff --git a/gdb/testsuite/gdb.mi/list-thread-groups-available.exp b/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
index 27224b5..3ab2cab 100644
--- a/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
+++ b/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
@@ -22,6 +22,7 @@
 
 # Support for XML is needed to run this test.
 require allow_xml_test
+require can_spawn_for_attach
 
 if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug}] != "" } {
     untested "failed to compile"
@@ -32,8 +33,6 @@
     return
 }
 
-require can_spawn_for_attach
-
 set string_re {(?:[^\\"]|\\.)*}
 
 set id_re "id=\"$decimal\""