`zig build test-cases --watch` does not pick up changes to source files

[andrewrk]

Extracted from #20580.

Currently the test-cases test suite walks the filesystem during the configure phase, loads the source files into memory, and adds WriteFile steps to write out the source code always to tmp.zig files, using those files as the inputs to Compile steps.

This issue is to rework the test cases to create the Compile steps directly using the source files from test/cases/*. This will make the file system watching mechanism pick up the changes properly.

Related:

• ability to register file system input dependencies during the configure phase #20605