[libc][stdlib] Simplify getenv_test by using inline_strcmp instead of custom helper

Replace the custom `my_streq` helper function with LLVM libc's
`inline_strcmp` utility from `src/string/memory_utils/inline_strcmp.h`.

Changes:
- Remove 18-line custom `my_streq` implementation
- Use `inline_strcmp` with a simple comparator lambda for string comparisons
- Replace `my_streq(..., nullptr)` checks with direct `== nullptr` comparisons
- Maintain identical test coverage while reducing code duplication

Benefits:
- Uses existing, well-tested LLVM libc infrastructure
- Clearer test assertions with standard comparison functions
- More concise code (reduced from ~48 to ~33 lines)
- Consistent with LLVM libc coding practices

The test continues to verify:
- Empty string handling
- Invalid name handling ('=')
- Missing environment variable queries
- Correct retrieval of existing variables (FRANCE, GERMANY, PATH)
- Partial name matching behavior (FRANC vs FRANCE, FRANCE1 vs FRANCE)

RE apply https://github.com/llvm/llvm-project/pull/133643/commits#top

Reverts llvm/llvm-project#133643

* Relates to: https://github.com/llvm/llvm-project/issues/119281

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

This part of the effort to make all test related pieces into the `test`
directory. This helps is excluding test related pieces in a straight
forward manner if LLVM_INCLUDE_TESTS is OFF. Future patches will also move
the MPFR wrapper and testutils into the 'test' directory.

This reverts commit 2846c2bb4fa4e50b2eb4ff4231825d73840c8c1c. The reason
for the disable is not relevant anymore.

This is blocking fullbuild bot.

All existing loader tests are switched to an integration test added with
the new rule. Also, the getenv test is now enabled as an integration test.

All loader tests have been moved to test/integration. Also, the simple
checker library for the previous loader tests has been moved to a
separate directory of its own.

A follow up change will perform more cleanup of the loader CMake rules
to eliminate now redundent options.

Reviewed By: lntue, michaelrj

Differential Revision: https://reviews.llvm.org/D122266