Check if Bundler is defined

	---
	 tool/lib/leakchecker.rb | 2 +-
	 1 file changed, 1 insertion(+), 1 deletion(-)

	Check if `Bundler::EnvironmentPreserver` is defined

	Only `Bundler` might be defined.  `EnvironmentPreserver` and its
	`BUNDLER_PREFIX` would be defined together in the same file.
	---
	 tool/lib/leakchecker.rb | 5 ++++-
	 1 file changed, 4 insertions(+), 1 deletion(-)

Bundler's backups changes environment variables starting with
BUNDLER_ORIG_. This causes a lot of noise in tests as the leakchecker
reports them as changed.

and /dev/urandom intentionally.  OpenSSL::PKey::RSA.new opens the two
random generators and keeps the file descriptors.

https://github.com/openssl/openssl/blob/93f99b681ab5a1cf7062053323e09b0cad5ff854/crypto/rand/rand_unix.c#L674

They are detected by the LeakChecker as fd leak, but it is intentional.

http://rubyci.s3.amazonaws.com/graviton2/ruby-master/log/20200526T160005Z.log.html.gz
```
[  597/20199] DRbTests::TestDRbSSLAry#test_01 = 0.29 s
Leaked file descriptor: DRbTests::TestDRbSSLAry#test_01: 8 #<File::Stat dev=0x6, ino=11, mode=020666, nlink=1, uid=0, gid=0, rdev=0x109, size=0, blksize=4096, blocks=0, atime=2020-05-23 14:45:13.751999995 +0000, mtime=2020-05-23 14:45:13.751999995 +0000, ctime=2020-05-23 14:45:13.751999995 +0000>
Leaked file descriptor: DRbTests::TestDRbSSLAry#test_01: 9 #<File::Stat dev=0x6, ino=10, mode=020666, nlink=1, uid=0, gid=0, rdev=0x108, size=0, blksize=4096, blocks=0, atime=2020-05-23 14:45:13.755999995 +0000, mtime=2020-05-23 14:45:13.755999995 +0000, ctime=2020-05-23 14:45:13.755999995 +0000>
```

When multiple autoclose IO objects are leaked too.