Fix browser not found err in TestBrowserPermissions

Keeping the expected error message to contain a minimal part of the
error so that if we change the error again, this test won't bail.

Related: grafana/xk6-browser#1136

cmd/tests/cmd_run_test.go

@@ -2204,7 +2204,7 @@ func TestBrowserPermissions(t *testing.T) {
 				},
 			}`,
 			expectedExitCode: 108,
-			expectedError:    "error building browser on IterStart: launching browser: exec: \"k6-browser-fake-cmd\": executable file not found",
+			expectedError:    "k6-browser-fake-cmd",
 		},
 	}