Add spec to Backtrace::APP_DIRS_PATTERN

[sl0thentr0py]

while working on the profiler, I noticed spec was missing here.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 ⚠️

Comparison is base (d45f720) 98.58% compared to head (3031948) 98.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2029      +/-   ##
==========================================
- Coverage   98.58%   98.56%   -0.02%     
==========================================
  Files         157      157              
  Lines       10127    10127              
==========================================
- Hits         9984     9982       -2     
- Misses        143      145       +2     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/backtrace.rb	96.61% <100.00%> (ø)
...try-ruby/spec/sentry/interfaces/stacktrace_spec.rb	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[st0012]

Not a blocker, but we probably should find another non-in-app trace to verify that check.

[sl0thentr0py]

added :)