Fix test build failure on Linux

[hannesbraun]

While trying to package MuJoCo for Alpine Linux, I encountered the following build failure when building the tests:

ninja: job failed: : && /usr/bin/c++ -Os -fstack-clash-protection -Wformat -Werror=format-security -D_GLIBCXX_ASSERTIONS=1 -D_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS=1 -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST -fno-plt -flto=auto -fno-fat-lto-objects -Wl,--as-needed,-O1,--sort-common -Wl,-z,pack-relative-relocs -Wl,--gc-sections test/engine/CMakeFiles/engine_solver_test.dir/engine_solver_test.cc.o -o bin/engine_solver_test  -Wl,-rpath,/tmp/src/mujoco-e4f4044659efb556e8010637142d63b7ed4463f1/build/lib  lib/libfixture.a  -lgmock  /usr/lib/libgtest_main.so.1.17.0  lib/libmujoco.so.3.7.0  /usr/lib/libabsl_raw_hash_set.so.2508.0.0  /usr/lib/libabsl_cord.so.2508.0.0  /usr/lib/libabsl_cordz_info.so.2508.0.0  /usr/lib/libabsl_cord_internal.so.2508.0.0  /usr/lib/libabsl_cordz_functions.so.2508.0.0  /usr/lib/libabsl_cordz_handle.so.2508.0.0  /usr/lib/libabsl_crc_cord_state.so.2508.0.0  /usr/lib/libabsl_crc32c.so.2508.0.0  /usr/lib/libabsl_crc_internal.so.2508.0.0  /usr/lib/libabsl_crc_cpu_detect.so.2508.0.0  /usr/lib/libabsl_str_format_internal.so.2508.0.0  /usr/lib/libabsl_hash.so.2508.0.0  /usr/lib/libabsl_city.so.2508.0.0  /usr/lib/libabsl_hashtablez_sampler.so.2508.0.0  /usr/lib/libabsl_exponential_biased.so.2508.0.0  /usr/lib/libabsl_synchronization.so.2508.0.0  /usr/lib/libabsl_graphcycles_internal.so.2508.0.0  /usr/lib/libabsl_kernel_timeout_internal.so.2508.0.0  /usr/lib/libabsl_stacktrace.so.2508.0.0  /usr/lib/libabsl_symbolize.so.2508.0.0  /usr/lib/libabsl_malloc_internal.so.2508.0.0  /usr/lib/libabsl_debugging_internal.so.2508.0.0  /usr/lib/libabsl_demangle_internal.so.2508.0.0  /usr/lib/libabsl_demangle_rust.so.2508.0.0  /usr/lib/libabsl_decode_rust_punycode.so.2508.0.0  /usr/lib/libabsl_utf8_for_code_point.so.2508.0.0  /usr/lib/libabsl_time.so.2508.0.0  /usr/lib/libabsl_strings.so.2508.0.0  /usr/lib/libabsl_strings_internal.so.2508.0.0  /usr/lib/libabsl_string_view.so.2508.0.0  /usr/lib/libabsl_throw_delegate.so.2508.0.0  /usr/lib/libabsl_civil_time.so.2508.0.0  /usr/lib/libabsl_int128.so.2508.0.0  /usr/lib/libabsl_time_zone.so.2508.0.0  /usr/lib/libabsl_tracing_internal.so.2508.0.0  /usr/lib/libabsl_base.so.2508.0.0  /usr/lib/libabsl_raw_logging_internal.so.2508.0.0  /usr/lib/libabsl_log_severity.so.2508.0.0  /usr/lib/libabsl_spinlock_wait.so.2508.0.0  /usr/lib/libgmock.so.1.17.0  /usr/lib/libgtest.so.1.17.0 && cd /tmp/src/mujoco-e4f4044659efb556e8010637142d63b7ed4463f1/build/test/engine && /usr/bin/cmake -D TEST_TARGET=engine_solver_test -D TEST_EXECUTABLE=/tmp/src/mujoco-e4f4044659efb556e8010637142d63b7ed4463f1/build/bin/engine_solver_test -D TEST_EXECUTOR= -D TEST_WORKING_DIR=/tmp/src/mujoco-e4f4044659efb556e8010637142d63b7ed4463f1/test -D TEST_EXTRA_ARGS= -D TEST_PROPERTIES= -D TEST_PREFIX= -D TEST_SUFFIX= -D TEST_FILTER= -D NO_PRETTY_TYPES=FALSE -D NO_PRETTY_VALUES=FALSE -D TEST_LIST=engine_solver_test_TESTS -D CTEST_FILE=/tmp/src/mujoco-e4f4044659efb556e8010637142d63b7ed4463f1/build/test/engine/engine_solver_test[1]_tests.cmake -D TEST_DISCOVERY_TIMEOUT=5 -D TEST_DISCOVERY_EXTRA_ARGS= -D TEST_XML_OUTPUT_DIR= -P /usr/share/cmake/Modules/GoogleTestAddTests.cmake
/usr/include/fortify/stdio.h: In function '__to_xstring.constprop':
/usr/include/fortify/stdio.h:82:28: error: inlining failed in call to 'always_inline' 'vsnprintf': function body can be overwritten at link time
   82 | _FORTIFY_FN(vsnprintf) int vsnprintf(char * _FORTIFY_POS0 __s, size_t __n,
      |                            ^
/usr/include/c++/15.2.0/ext/string_conversions.h:115:32: note: called from here
  115 |       const int __len = __convf(__s, __n, __fmt, __args);
      |                                ^

I was able to trace the error down to the std::to_string call, so I moved the string generation into the loop, eliminating the direct call to to_string. This might be a little less performant but since it only affects the tests, I thought that this is okay.