`Library/ENV` like `Library/Homebrew` is part of Homebrew basecode. It
should be able to be accessed during the `brew tests`.

By adding HOMEBREW_ENV_PATH variable, we allow test suit to locate these
codes.

Closes Homebrew/homebrew#48923.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>

Currently HOMEBREW_BREW_FILE is a String, while other of HOMEBREW_*
variables are all Pathname. This commit unifies them all as Pathname,
so it will not cause any confusion.

Closes Homebrew/homebrew#48872.

Signed-off-by: Xu Cheng <xucheng@me.com>

This fixes a regression introduced by 3f6a355, which caused
HOMEBREW_BREW_FILE to be unset while running the tests.

Patch provided by Xu Cheng.

The "apply" DSL method can be called from patch-do blocks to specify
the paths within an archive of the desired patch files, which will be
applied in the order in which they were supplied to the "apply" calls.
If "apply" isn't used, raise an error whenever the extracted directory
doesn't contain exactly one file.

The "apply" method can be called zero or more times within a patch-do
block with the following syntaxes supported:

    apply "single_apply"
    apply "multiple_apply_1", "multiple_apply_2"
    apply [array_of_apply]

If apply must be used, a single call using the second syntax above is
usually best practice. Each apply leaf should be the relative path to a
specific patch file in the extracted directory.

For example, if extracting this-v123-patches.tar.gz gives you

    this-123
    this-123/.DS_Store
    this-123/LICENSE.txt
    this-123/patches
    this-123/patches/A.diff
    this-123/patches/B.diff
    this-123/patches/C.diff
    this-123/README.txt

and you want to apply only B.diff and C.diff, then you need to use
"patches/B.diff" and "patches/C.diff" for the lowest-level apply leaves.

The code was provided by Xu Cheng. Any mistakes are mine.

Closes Homebrew/homebrew#47714.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>