[macOS] 2 tests fail

```
R version 4.3.0 (2023-04-21) -- "Already Tomorrow"
Copyright (C) 2023 The R Foundation for Statistical Computing
Platform: powerpc-apple-darwin10.8.0 (32-bit)

> testthat::test_check("PosteriorBootstrap")
Loading required package: PosteriorBootstrap
Welcome to PosteriorBootstrap, a parallel approach for adaptive non-parametric learning
[ FAIL 2 | WARN 0 | SKIP 3 | PASS 51 ]

══ Skipped tests (3) ═══════════════════════════════════════════════════════════
• On CRAN (3): 'test_anpl.R:116:3', 'test_anpl.R:123:3',
'test_german_data.R:39:3'

══ Failed tests ════════════════════════════════════════════════════════════════
── Error ('test_anpl.R:175:3'): Adaptive non-parametric learning with posterior samples works ──
Error in `dyn.load(libLFile)`: unable to load shared object '/opt/local/var/macports/build/_opt_PPCRosettaPorts_R_R-PosteriorBootstrap/R-PosteriorBootstrap/work/.tmp/RtmpfLsFou/file1633224efa530.so':
dlopen(/opt/local/var/macports/build/_opt_PPCRosettaPorts_R_R-PosteriorBootstrap/R-PosteriorBootstrap/work/.tmp/RtmpfLsFou/file1633224efa530.so, 6): Library not loaded: @rpath/libtbb.dylib
Referenced from: /opt/local/var/macports/build/_opt_PPCRosettaPorts_R_R-PosteriorBootstrap/R-PosteriorBootstrap/work/.tmp/RtmpfLsFou/file1633224efa530.so
Reason: image not found
Backtrace:
▆
1. └─rstan::stan_model(file = get_stan_file()) at test_anpl.R:175:2
2. └─rstan:::cxxfunctionplus(...)
3. ├─pkgbuild::with_build_tools(...)
4. └─inline::cxxfunction(...)
5. └─base::dyn.load(libLFile)
── Error ('test_anpl.R:175:3'): Adaptive non-parametric learning with posterior samples works ──
Error in `sink(type = "output")`: invalid connection
Backtrace:
▆
1. └─rstan::stan_model(file = get_stan_file()) at test_anpl.R:175:2
2. └─rstan:::cxxfunctionplus(...)
3. └─base::sink(type = "output")

[ FAIL 2 | WARN 0 | SKIP 3 | PASS 51 ]
Error: Test failures
Execution halted
```
The first error seems to be caused by usage of rpath in `@rpath/libtbb.dylib`; instead, absolute path should be used.

Start by reproducing the reported `testthat::test_check("PosteriorBootstrap")` run on macOS and inspect `test_anpl.R` at line 175, where `rstan::stan_model` triggers the failure. Trace the `@rpath/libtbb.dylib` load error and the subsequent invalid connection error; done means the two reported failures no longer occur in the test run.

Written by the indexing model from the issue text.