Add interleaved benchmark execution for test-suite

[aemerson]

Add interleaved benchmark execution for test-suite

This adds support for separating build and test phases, and running
benchmarks from multiple builds in an interleaved fashion to control
for environmental factors (ambient temperature, general system load etc).

New options:

• --build-only: Build tests without running them
• --test-prebuilt: Run tests from pre-built directory
• --build-dir: Specify build directory (used with --test-prebuilt)
• --exec-interleaved-builds: Comma-separated list of builds to interleave

Usage:

1. Build two compiler versions:
   lnt runtest test-suite --build-only
   --sandbox /tmp/sandbox-a
   --cc /path/to/clang-a
   --test-suite ~/llvm-test-suite
   ...

   lnt runtest test-suite --build-only
   --sandbox /tmp/sandbox-b
   --cc /path/to/clang-b
   --test-suite ~/llvm-test-suite
   ...

2. Run with interleaved execution:
   lnt runtest test-suite
   --sandbox /tmp/results
   --exec-interleaved-builds /tmp/sandbox-a/build,/tmp/sandbox-b/build
   --exec-multisample 3

   This runs tests in the pattern:

   • Sample 0: build-a -> build-b
   • Sample 1: build-a -> build-b
   • Sample 2: build-a -> build-b

   Temporal interleaving controls for environmental changes that could
   bias results toward one build.

Or, test single build:
lnt runtest test-suite --test-prebuilt
--build-dir /tmp/sandbox-a/build
--exec-multisample 5

Reports are written to each build directory (report.json,
test-results.xunit.xml, test-results.csv).

Generalize --build-dir to work with all build modes

The --build-dir option now works with all build modes, not just
--test-prebuilt. It can be used to specify a custom build directory
location, overriding the default sandbox/build or timestamped directory.

[aemerson]

• Add interleaved benchmark execution for test-suite #106 👈 (View in Graphite)
• main

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