Prior to this change, each invocation of `loadStandardTestFiles` would
load the necessary files from disk. This function is typically called
at the top-level of a test module in order to share the result across
tests. The `//packages/compiler-cli/test/ngtsc` target has 8 modules
where this call occurs, each loading their own copy of
`node_modules/typescript` which is ~60MB in size, so the memory overhead
used to be significant. This commit loads the individual packages into
a standalone `Folder` and mounts this folder into the filesystem of
standard test files, such that all file contents are no longer
duplicated in memory.
PR Close#38909
e790c8547e