Testsuite: T7043: make build/manifest.json optional

Support loading and testing random ISO images without the need for
build/manifest.json. If the file is detected - tests are run. If file is
not found - those tests are skipped.

scripts/check-qemu-install

@@ -105,11 +105,13 @@ with open('data/defaults.toml', 'rb') as f:
     vyos_defaults = tomli.load(f)
 
 # This is what we got from the build
-with open('build/manifest.json', 'rb') as f:
-    manifest = json.load(f)
+manifest_file = 'build/manifest.json'
+if os.path.isfile(manifest_file):
+    with open('build/manifest.json', 'rb') as f:
+        manifest = json.load(f)
 
-vyos_version = manifest['build_config']['version']
-vyos_codename = manifest['build_config']['release_train']
+    vyos_version = manifest['build_config']['version']
+    vyos_codename = manifest['build_config']['release_train']
 
 class StreamToLogger(object):
     """
@@ -558,10 +560,11 @@ try:
     #################################################
     c.sendline('lsb_release --short --id 2>/dev/null')
     c.expect('VyOS')
-    c.sendline('lsb_release --short --release 2>/dev/null')
-    c.expect(vyos_version)
-    c.sendline('lsb_release --short --codename 2>/dev/null')
-    c.expect(vyos_codename)
+    if os.path.isfile(manifest_file):
+        c.sendline('lsb_release --short --release 2>/dev/null')
+        c.expect(vyos_version)
+        c.sendline('lsb_release --short --codename 2>/dev/null')
+        c.expect(vyos_codename)
 
     # Ensure ephemeral key is loaded
     vyos_kernel_key = 'VyOS build time autogenerated kernel key'