build: properly skip tests

I got this failure on FreeBSD:

shunloadtest.c: In function 'main':
shunloadtest.c:150: error: 'EXIT_AM_SKIP' undeclared (first use in this function)

but inspection showed several other problems, all fixed here.

* tests/domainsnapshotxml2xmltest.c [!WITH_QEMU]: Ensure
EXIT_AM_SKIP is defined.
* tests/esxutilstest.c [!WITH_ESX]: Likewise.
* tests/openvzutilstest.c [!WITH_OPENVZ]: Likewise.
* tests/qemuargv2xmltest.c [!WITH_QEMU]: Likewise.
* tests/qemuhelptest.c [!WITH_QEMU]: Likewise.
* tests/qemuxml2argvtest.c [!WITH_QEMU]: Likewise.
* tests/qemuxml2xmltest.c [!WITH_QEMU]: Likewise.
* tests/qemuxmlnstest.c [!WITH_QEMU]: Likewise.
* tests/shunloadtest.c [!linux]: Likewise.
* tests/vmx2xmltest.c [!WITH_VMX]: Likewise.
* tests/xml2vmxtest.c [!WITH_VMX]: Likewise.

tests/domainsnapshotxml2xmltest.c

@@ -118,6 +118,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int
 main(void)

tests/esxutilstest.c

@@ -279,6 +279,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int main(void)
 {

tests/openvzutilstest.c

@@ -160,6 +160,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int main(void)
 {

tests/qemuargv2xmltest.c

@@ -243,6 +243,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int
 main(void)

tests/qemuhelptest.c

@@ -578,6 +578,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int main(void)
 {

tests/qemuxml2argvtest.c

@@ -669,6 +669,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int main(void)
 {

tests/qemuxml2xmltest.c

@@ -211,6 +211,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int
 main(void)

tests/qemuxmlnstest.c

@@ -284,6 +284,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int main(void)
 {

tests/shunloadtest.c

@@ -144,6 +144,7 @@ int main(int argc ATTRIBUTE_UNUSED, char **argv)
 }
 
 #else
+# include "testutils.h"
 
 int main(void)
 {

tests/vmx2xmltest.c

@@ -289,6 +289,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int main(void)
 {

tests/xml2vmxtest.c

@@ -301,6 +301,7 @@ mymain(void)
 VIRT_TEST_MAIN(mymain)
 
 #else
+# include "testutils.h"
 
 int main(void)
 {