Enable extended regex in assert_file_contains

Signed-off-by: David Bibby <davidsbibby@gmail.com>

src/file.bash

diff --git a/src/file.bash b/src/file.bash
index a79ff09..5fe0ef5 100644
--- a/src/file.bash
+++ b/src/file.bash
@@ -551,7 +551,7 @@
 assert_file_contains() {
   local -r file="$1"
   local -r regex="$2"
-  if ! grep -q "$regex" "$file"; then
+  if ! grep -E -q "$regex" "$file"; then
     local -r rem="${BATSLIB_FILE_PATH_REM-}"
     local -r add="${BATSLIB_FILE_PATH_ADD-}"
     batslib_print_kv_single 4 'path' "${file/$rem/$add}" \