tests: avoid side effects of $SHELL environment variable

Since non interactive shells don't generally set $SHELL,
its value is propagated through the tests and may cause issues;
for example if $SHELL implicitly adjusts $PATH when run.
Instead we set $SHELL to that determined by the posix-shell module,
and use that consistently for all test sub scripts,
including those created thorugh the `split --filter` command.

* tests/local.mk: Explicitly set $SHELL to $(PREFERABLY_POSIX_SHELL)
which defaults to $CONFIG_SHELL and thus usually /bin/sh.
* tests/envvar-check: Remove bash environment variables with
side effects, in case /bin/bash was selected for $SHELL.
* tests/misc/help-version.sh: Remove redundant initialization of $SHELL.
* tests/install/strip-program.sh: Use $SHELL for sub script.
* tests/misc/sort-compress-hang.sh: Likewise.
* tests/misc/sort-compress-proc.sh: Likewise.
* tests/misc/sort-compress.sh: Likewise.
* tests/misc/timeout-group.sh: Likewise.
* tests/rm/fail-eperm.xpl: Remove redundant elision of bash env vars.
* tests/misc/pwd-long.sh: Likewise.

tests/envvar-check

@@ -28,12 +28,14 @@ vars='
   _STDBUF_E
   _STDBUF_I
   _STDBUF_O
+  BASH_ENV
   BLOCKSIZE
   BLOCK_SIZE
   CDPATH
   COLUMNS
   DF_BLOCK_SIZE
   DU_BLOCK_SIZE
+  ENV
   LANGUAGE
   LS_BLOCK_SIZE
   LS_COLORS