Revert "tests/libvm: write rpm build logs to file"

This reverts commit 1ef259ed76.

Basically, for historical POSIX compatibility, `errexit` doesn't work at
all inside functions called from an if-statement/boolean context. This
is something I had already learned (and forgotten) when hacking on PAPR.

There are ways around this, but it's just not worth the added complex
shell goop for what it gives us.

http://mywiki.wooledge.org/BashFAQ/105

Closes: #1215
Approved by: cgwalters

tests/common/libvm.sh

@@ -347,20 +347,14 @@ vm_assert_status_jq() {
 
 # Like build_rpm, but also sends it to the VM
 vm_build_rpm() {
-    if ! build_rpm "$@" &> build.txt; then
-      cat build.txt
-      assert_not_reached "failed to build RPM"
-    fi
+    build_rpm "$@"
     vm_send_test_repo
 }
 
 # Like vm_build_rpm but takes a yumrepo mode
 vm_build_rpm_repo_mode() {
     mode=$1; shift
-    if ! build_rpm "$@" &> build.txt; then
-      cat build.txt
-      assert_not_reached "failed to build RPM"
-    fi
+    build_rpm "$@"
     vm_send_test_repo $mode
 }