Handle slang-test command comments better (#8363)

Before this PR only the following was a valid line without any
white-space character nor additional `/` character,
```
//TEST:
```

This PR is to allow slang-test to handle the following variants of the
test command comments,
```
///TEST:
// TEST:
//    TEST:
////// TEST:
```

This PR revealed a regression on two tests:
- tests/cpp-compiler/c-compile-shared-library.c (cpu)
- tests/cpp-compiler/cpp-compile-shared-library.cpp (cpu)

They are disabled as a part of this PR.
And there is a new github issue to track it later,
- https://github.com/shader-slang/slang/issues/8362

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