[fs] Decrease the minimum disk size for large tests.

ZX-2383 #comment In Progress
Test: fs-test, with both a ramdisk and with a 100 MB disk.

Change-Id: Ia58fa6503750b4ebc0d21a738d703b5fad74a302

[fs-test] Fix disk usage for fs_resize_tests_data.

Instead of assuming that a given number of files will fill the disk,
look at the actual disk size.

ZX-2383 #comment fix failing tests.
Test: fs-test

Change-Id: I4c6ca4abe6762fad0451c4c8839deeb5234665bd

[fs] Update filesystem tests to use custom disk sizes.

- Allow tests to specify a minimum disk size and
custom FVM slice size.
- If running against a ramdisk, create a disk using
the requested parameters.
- If running against a real disk, verify it has at
least as much space as the ramdisk would before
executing the test.
- Update the "default disk size" to be significantly
smaller, with manual annotations for tests that require
larger disks. This will allow the fs-test suite to run
against smaller disks, but without failure.

ZX-1604 #comment In Progress

Test: Ramdisk and real disks
$ /boot/test/fs-test -f minfs
$ /boot/test/fs-test -f minfs -d /dev/class/block/000

Change-Id: If139bb947734ce411af839b100c8557ea9378a91

[utest] Remove empty-string "msg" args from EXPECT/ASSERT calls in C++

The "msg" argument is optional in C++. A number of instances of
empty-string "msg" arguments have appeared since I last removed them.

This cleanup was automated.

ZX-905

Change-Id: I434d129836642216588f110d1cb161859ff3efea

[minfs] Enable asynchronous writeback for MinFS

MinFS spawns a background thread which manages all writes
to disk. Rather than blocking, waiting for a write to complete
in an I/O operation (like write, truncate, create, etc) the
modified blocks are copied into an in-memory buffer, which the
background thread dumps to disk.

ZX-1285 #done

Change-Id: Iccdb70da02db450551091d46c897de770f7559ce

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[zircon][mxtl->fbl] Rename mxtl to fbl

Change-Id: Ie21b6498e1bfb0a7fa0315e40b9e5c3ee78646be

[minfs][fs] Make MinFS automatically resize self on FVM

Change-Id: I3067e72adfa002a948ad6f9ba9c2cfb646adde90