[libc++] Improves pragma system_header test.

The number of spaces between `#` and `pragma` can differ due to
different indention levels in the preprocessor directives. Therefore
allow any number of spaces.

The test used to put an exclamation mark in its diagnostic. This adds
little benefit and only makes it harder to copy the offending filename.
As drive-by this exclamation mark has been removed.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D129047