test(python): cover --junit-xml= equals-empty rejection

The table had 3 of 4 cells in the junitxml 2×2 (spelling × form):
--junitxml=value, --junitxml=, --junit-xml=value were all tested.
The hyphenated equals-empty form --junit-xml= was not, leaving a
symmetry gap. The existing prefix check already rejects it; this
pins that behaviour against future regression.

pkg/python/test_test.go

@@ -100,6 +100,12 @@ func TestRunTests(t *testing.T) {
 			wantErr:     true,
 			errContains: "--junit-xml",
 		},
+		{
+			name:        "conflicting --junit-xml= (hyphenated equals-empty form) in testOptions is rejected",
+			testOptions: []string{"--junit-xml="},
+			wantErr:     true,
+			errContains: "--junit-xml",
+		},
 		{
 			name:        "conflicting --cov-report=xml in testOptions is rejected",
 			testOptions: []string{"--cov-report=xml:other.xml"},