virshtest: Test against output files ("echo" tests)

Managing output files is much simpler especially with
VIR_TEST_REGENERATE_OUTPUT compared to putting the expected string blobs
into the C source file.

For now the output is tested both against the hardcoded strings as well
as the output files.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tests/virshtestdata/multiple-commands-2.out

@@ -0,0 +1,2 @@
+a
+b