Add dosfstools to Linux install docs for tests
This is needed or all of the tests that run `fsck.fat` will fail.
This works on Ubuntu 20.04:
```sh
-sudo apt-get install build-essential autoconf pkg-config libtool mtools unzip zip help2man libconfuse-dev libarchive-dev xdelta3
+sudo apt-get install build-essential autoconf pkg-config libtool mtools unzip zip help2man libconfuse-dev libarchive-dev xdelta3 dosfstools
./configure
make -j4
make check
@@ -37,7 +37,7 @@ At least the following are required for building and running the regression
tests:
```sh
-sudo apt-get install build-essential autoconf pkg-config libtool mtools unzip zip help2man xdelta3
+sudo apt-get install build-essential autoconf pkg-config libtool mtools unzip zip help2man xdelta3 dosfstools
```
## Static build
